diff --git a/Puc/v4/Factory.php b/Puc/v4/Factory.php
index 7293299..147150e 100644
--- a/Puc/v4/Factory.php
+++ b/Puc/v4/Factory.php
@@ -23,13 +23,13 @@ if ( !class_exists('Puc_v4_Factory', false) ):
 		 *
 		 * @see PluginUpdateChecker::__construct()
 		 *
-		 * @param string $metadataUrl The URL of the metadata file, or a GitHub repository, etc.
+		 * @param string $metadataUrl The URL of the metadata file, a GitHub repository, or another supported update source.
 		 * @param string $fullPath Full path to the main plugin file or to the theme directory.
 		 * @param string $slug Custom slug. Defaults to the name of the main plugin file or the theme directory.
 		 * @param int $checkPeriod How often to check for updates (in hours).
 		 * @param string $optionName Where to store book-keeping info about update checks.
 		 * @param string $muPluginFile The plugin filename relative to the mu-plugins directory.
-		 * @return Puc_v4_Plugin_UpdateChecker|Puc_v4_Theme_UpdateChecker
+		 * @return Puc_v4_Plugin_UpdateChecker|Puc_v4_Theme_UpdateChecker|Puc_v4_Vcs_BaseChecker
 		 */
 		public static function buildUpdateChecker($metadataUrl, $fullPath, $slug = '', $checkPeriod = 12, $optionName = '', $muPluginFile = '') {
 			$fullPath = wp_normalize_path($fullPath);
@@ -68,31 +68,63 @@ if ( !class_exists('Puc_v4_Factory', false) ):
 				}
 			}
 
-			$class = null;
+			$checkerClass = null;
+			$apiClass = null;
 			if ( empty($service) ) {
 				//The default is to get update information from a remote JSON file.
-				$class = $type . '_UpdateChecker';
+				$checkerClass = $type . '_UpdateChecker';
 			} else {
-				$class = $service . '_' . $type . 'UpdateChecker';
+				//You can also use a VCS repository like GitHub.
+				$checkerClass = 'Vcs_' . $type . 'UpdateChecker';
+				$apiClass = $service . 'Api';
 			}
 
-			if ( !isset(self::$classVersions[$class][self::$greatestCompatVersion]) ) {
+			$checkerClass = self::getCompatibleClass($checkerClass);
+			if ( !$checkerClass ) {
 				trigger_error(
 					sprintf(
-						'PUC %s does not support updates for %ss hosted on %s',
+						'PUC %s does not support updates for %ss %s',
 						htmlentities(self::$greatestCompatVersion),
 						strtolower($type),
-						$service
+						$service ? ('hosted on ' . htmlentities($service)) : 'using JSON metadata'
 					),
 					E_USER_ERROR
 				);
 				return null;
 			}
 
-			$class = self::$classVersions[$class][self::$greatestCompatVersion];
-			return new $class($metadataUrl, $id, $slug, $checkPeriod, $optionName, $muPluginFile);
+			if ( !isset($apiClass) ) {
+				//Plain old update checker.
+				return new $checkerClass($metadataUrl, $id, $slug, $checkPeriod, $optionName, $muPluginFile);
+			} else {
+				//VCS checker + an API client.
+				$apiClass = self::getCompatibleClass($apiClass);
+				if ( !$apiClass ) {
+					trigger_error(sprintf(
+						'PUC %s does not support %s',
+						htmlentities(self::$greatestCompatVersion),
+						htmlentities($service)
+					), E_USER_ERROR);
+					return null;
+				}
+
+				return new $checkerClass(
+					new $apiClass($metadataUrl),
+					$id,
+					$slug,
+					$checkPeriod,
+					$optionName,
+					$muPluginFile
+				);
+			}
 		}
 
+		/**
+		 * Check if the path points to something inside the "plugins" or "mu-plugins" directories.
+		 *
+		 * @param string $absolutePath
+		 * @return bool
+		 */
 		protected static function isPluginFile($absolutePath) {
 			$pluginDir = wp_normalize_path(WP_PLUGIN_DIR);
 			$muPluginDir = wp_normalize_path(WPMU_PLUGIN_DIR);
@@ -101,6 +133,20 @@ if ( !class_exists('Puc_v4_Factory', false) ):
 			return (strpos($absolutePath, $pluginDir) === 0) || (strpos($absolutePath, $muPluginDir) === 0);
 		}
 
+		/**
+		 * Get the latest version of the specified class that has the same major version number
+		 * as this factory class.
+		 *
+		 * @param string $class Partial class name.
+		 * @return string|null Full class name.
+		 */
+		protected static function getCompatibleClass($class) {
+			if ( isset(self::$classVersions[$class][self::$greatestCompatVersion]) ) {
+				return self::$classVersions[$class][self::$greatestCompatVersion];
+			}
+			return null;
+		}
+
 		/**
 		 * Get the specific class name for the latest available version of a class.
 		 *
diff --git a/Puc/v4/GitHub/PluginUpdateChecker.php b/Puc/v4/GitHub/PluginUpdateChecker.php
deleted file mode 100644
index 31c0c9c..0000000
--- a/Puc/v4/GitHub/PluginUpdateChecker.php
+++ /dev/null
@@ -1,224 +0,0 @@
-<?php
-
-if ( !class_exists('Puc_v4_GitHub_PluginUpdateChecker', false) ):
-
-class Puc_v4_GitHub_PluginUpdateChecker extends Puc_v4_Plugin_UpdateChecker {
-	/**
-	 * @var string Either a fully qualified repository URL, or just "user/repo-name".
-	 */
-	protected $repositoryUrl;
-
-	/**
-	 * @var string The branch to use as the latest version. Defaults to "master".
-	 */
-	protected $branch = 'master';
-
-	/**
-	 * @var string GitHub authentication token. Optional.
-	 */
-	protected $accessToken;
-
-	/**
-	 * @var Puc_v4_GitHub_Api
-	 */
-	protected $api;
-
-	public function __construct(
-		$repositoryUrl,
-		$pluginFile,
-		$slug = '',
-		$checkPeriod = 12,
-		$optionName = '',
-		$muPluginFile = ''
-	) {
-		$this->repositoryUrl = $repositoryUrl;
-		parent::__construct($repositoryUrl, $pluginFile, $slug, $checkPeriod, $optionName, $muPluginFile);
-	}
-
-	/**
-	 * Set the GitHub branch to use for updates. Defaults to 'master'.
-	 *
-	 * @param string $branch
-	 * @return $this
-	 */
-	public function setBranch($branch) {
-		$this->branch = empty($branch) ? 'master' : $branch;
-		return $this;
-	}
-
-	/**
-	 * Retrieve details about the latest plugin version from GitHub.
-	 *
-	 * @param array $unusedQueryArgs Unused.
-	 * @return Puc_v4_Plugin_Info
-	 */
-	public function requestInfo($unusedQueryArgs = array()) {
-		$api = $this->api = new Puc_v4_GitHub_Api($this->repositoryUrl, $this->accessToken);
-
-		$info = new Puc_v4_Plugin_Info();
-		$info->filename = $this->pluginFile;
-		$info->slug = $this->slug;
-
-		$this->setInfoFromHeader($this->getPluginHeader(), $info);
-
-		//Figure out which reference (tag or branch) we'll use to get the latest version of the plugin.
-		$updateSource = $this->chooseReference();
-		if ( $updateSource ) {
-			$ref = $updateSource->name;
-			$info->version = $updateSource->version;
-			$info->last_updated = $updateSource->updated;
-			$info->download_url = $updateSource->downloadUrl;
-
-			if ( !empty($updateSource->changelog) ) {
-				$info->sections['changelog'] = $updateSource->changelog;
-			}
-			if ( isset($updateSource->downloadCount) ) {
-				$info->downloaded = $updateSource->downloadCount;
-			}
-		} else {
-			return null;
-		}
-
-		if ( !empty($info->download_url) && !empty($this->accessToken) ) {
-			$info->download_url = add_query_arg('access_token', $this->accessToken, $info->download_url);
-		}
-
-		//Get headers from the main plugin file in this branch/tag. Its "Version" header and other metadata
-		//are what the WordPress install will actually see after upgrading, so they take precedence over releases/tags.
-		$mainPluginFile = basename($this->pluginFile);
-		$remotePlugin = $api->getRemoteFile($mainPluginFile, $ref);
-		if ( !empty($remotePlugin) ) {
-			$remoteHeader = $this->getFileHeader($remotePlugin);
-			$this->setInfoFromHeader($remoteHeader, $info);
-		}
-
-		//Try parsing readme.txt. If it's formatted according to WordPress.org standards, it will contain
-		//a lot of useful information like the required/tested WP version, changelog, and so on.
-		if ( $this->readmeTxtExistsLocally() ) {
-			$this->setInfoFromRemoteReadme($ref, $info);
-		}
-
-		//The changelog might be in a separate file.
-		if ( empty($info->sections['changelog']) ) {
-			$info->sections['changelog'] = $api->getRemoteChangelog($ref, dirname($this->getAbsolutePath()));
-			if ( empty($info->sections['changelog']) ) {
-				$info->sections['changelog'] = __('There is no changelog available.', 'plugin-update-checker');
-			}
-		}
-
-		if ( empty($info->last_updated) ) {
-			//Fetch the latest commit that changed the tag/branch and use it as the "last_updated" date.
-			$info->last_updated = $api->getLatestCommitTime($ref);
-		}
-
-		$info = apply_filters($this->getUniqueName('request_info_result'), $info, null);
-		return $info;
-	}
-
-	/**
-	 * @return Puc_v4_VcsReference|null
-	 */
-	protected function chooseReference() {
-		$api = $this->api;
-		$updateSource = null;
-
-		if ( $this->branch === 'master' ) {
-			//Use the latest release.
-			$updateSource = $api->getLatestRelease();
-			if ( $updateSource === null ) {
-				//Failing that, use the tag with the highest version number.
-				$updateSource = $api->getLatestTag();
-			}
-		}
-		//Alternatively, just use the branch itself.
-		if ( empty($ref) ) {
-			$updateSource = $api->getBranch($this->branch);
-		}
-
-		return $updateSource;
-	}
-
-	/**
-	 * Set the access token that will be used to make authenticated GitHub API requests.
-	 *
-	 * @param string $accessToken
-	 * @return $this
-	 */
-	public function setAccessToken($accessToken) {
-		$this->accessToken = $accessToken;
-		return $this;
-	}
-
-	/**
-	 * Copy plugin metadata from a file header to a PluginInfo object.
-	 *
-	 * @param array $fileHeader
-	 * @param Puc_v4_Plugin_Info $pluginInfo
-	 */
-	protected function setInfoFromHeader($fileHeader, $pluginInfo) {
-		$headerToPropertyMap = array(
-			'Version' => 'version',
-			'Name' => 'name',
-			'PluginURI' => 'homepage',
-			'Author' => 'author',
-			'AuthorName' => 'author',
-			'AuthorURI' => 'author_homepage',
-
-			'Requires WP' => 'requires',
-			'Tested WP' => 'tested',
-			'Requires at least' => 'requires',
-			'Tested up to' => 'tested',
-		);
-		foreach ($headerToPropertyMap as $headerName => $property) {
-			if ( isset($fileHeader[$headerName]) && !empty($fileHeader[$headerName]) ) {
-				$pluginInfo->$property = $fileHeader[$headerName];
-			}
-		}
-
-		if ( !empty($fileHeader['Description']) ) {
-			$pluginInfo->sections['description'] = $fileHeader['Description'];
-		}
-	}
-
-	/**
-	 * Copy plugin metadata from the remote readme.txt file.
-	 *
-	 * @param string $ref GitHub tag or branch where to look for the readme.
-	 * @param Puc_v4_Plugin_Info $pluginInfo
-	 */
-	protected function setInfoFromRemoteReadme($ref, $pluginInfo) {
-		$readme = $this->api->getRemoteReadme($ref);
-		if ( empty($readmeTxt) ) {
-			return;
-		}
-
-		if ( isset($readme['sections']) ) {
-			$pluginInfo->sections = array_merge($pluginInfo->sections, $readme['sections']);
-		}
-		if ( !empty($readme['tested_up_to']) ) {
-			$pluginInfo->tested = $readme['tested_up_to'];
-		}
-		if ( !empty($readme['requires_at_least']) ) {
-			$pluginInfo->requires = $readme['requires_at_least'];
-		}
-
-		if ( isset($readme['upgrade_notice'], $readme['upgrade_notice'][$pluginInfo->version]) ) {
-			$pluginInfo->upgrade_notice = $readme['upgrade_notice'][$pluginInfo->version];
-		}
-	}
-
-	/**
-	 * Check if the currently installed version has a readme.txt file.
-	 *
-	 * @return bool
-	 */
-	protected function readmeTxtExistsLocally() {
-		$pluginDirectory = dirname($this->pluginAbsolutePath);
-		if ( empty($this->pluginAbsolutePath) || !is_dir($pluginDirectory) || ($pluginDirectory === '.') ) {
-			return false;
-		}
-		return is_file($pluginDirectory . '/readme.txt');
-	}
-}
-
-endif;
\ No newline at end of file
diff --git a/Puc/v4/GitHub/ThemeUpdateChecker.php b/Puc/v4/GitHub/ThemeUpdateChecker.php
deleted file mode 100644
index 4721759..0000000
--- a/Puc/v4/GitHub/ThemeUpdateChecker.php
+++ /dev/null
@@ -1,99 +0,0 @@
-<?php
-if ( !class_exists('Puc_v4_GitHub_ThemeUpdateChecker', false) ):
-
-	class Puc_v4_GitHub_ThemeUpdateChecker extends Puc_v4_Theme_UpdateChecker {
-		protected $repositoryUrl;
-		protected $branch = 'master';
-		protected $accessToken;
-
-		public function __construct($repositoryUrl, $stylesheet = null, $customSlug = null, $checkPeriod = 12, $optionName = '') {
-			$this->repositoryUrl = $repositoryUrl;
-			parent::__construct($repositoryUrl, $stylesheet, $customSlug, $checkPeriod, $optionName);
-		}
-
-		public function requestUpdate() {
-			$api = new Puc_v4_GitHub_Api($this->repositoryUrl, $this->accessToken);
-
-			$update = new Puc_v4_Theme_Update();
-			$update->slug = $this->slug;
-
-			//Figure out which reference (tag or branch) we'll use to get the latest version of the theme.
-			$ref = $this->branch;
-			if ( $this->branch === 'master' ) {
-				//Use the latest release.
-				$release = $api->getLatestRelease();
-				if ( $release !== null ) {
-					$ref = $release->name;
-					$update->version = ltrim($release->name, 'v'); //Remove the "v" prefix from "v1.2.3".
-					$update->download_url = $release->downloadUrl;
-				} else {
-					//Failing that, use the tag with the highest version number.
-					$tag = $api->getLatestTag();
-					if ( $tag !== null ) {
-						$ref = $tag->name;
-						$update->version = $tag->version;
-						$update->download_url = $tag->downloadUrl;
-					}
-				}
-			}
-
-			if ( empty($update->download_url) ) {
-				$update->download_url = $api->buildArchiveDownloadUrl($ref);
-			} else if ( !empty($this->accessToken) ) {
-				$update->download_url = add_query_arg('access_token', $this->accessToken, $update->download_url);
-			}
-
-			//Get headers from the main stylesheet in this branch/tag. Its "Version" header and other metadata
-			//are what the WordPress install will actually see after upgrading, so they take precedence over releases/tags.
-			$remoteStylesheet = $api->getRemoteFile('style.css', $ref);
-			if ( !empty($remoteStylesheet) ) {
-				$remoteHeader = $this->getFileHeader($remoteStylesheet);
-				if ( !empty($remoteHeader['Version']) ) {
-					$update->version = $remoteHeader['Version'];
-				}
-				if ( !empty($remoteHeader['ThemeURI']) ) {
-					$update->details_url = $remoteHeader['ThemeURI'];
-				}
-			}
-
-			//The details URL defaults to the Theme URI header or the repository URL.
-			if ( empty($update->details_url) ) {
-			 	$update->details_url = $this->theme->get('ThemeURI');
-			}
-			if ( empty($update->details_url) ) {
-				$update->details_url = $this->repositoryUrl;
-			}
-
-			if ( empty($update->version) ) {
-				//It looks like we didn't find a valid update after all.
-				$update = null;
-			}
-
-			$update = $this->filterUpdateResult($update);
-			return $update;
-		}
-
-		/**
-		 * Set the GitHub branch to use for updates. Defaults to 'master'.
-		 *
-		 * @param string $branch
-		 * @return $this
-		 */
-		public function setBranch($branch) {
-			$this->branch = empty($branch) ? 'master' : $branch;
-			return $this;
-		}
-
-		/**
-		 * Set the access token that will be used to make authenticated GitHub API requests.
-		 *
-		 * @param string $accessToken
-		 * @return $this
-		 */
-		public function setAccessToken($accessToken) {
-			$this->accessToken = $accessToken;
-			return $this;
-		}
-	}
-
-endif;
\ No newline at end of file
diff --git a/Puc/v4/VcsApi.php b/Puc/v4/Vcs/Api.php
similarity index 62%
rename from Puc/v4/VcsApi.php
rename to Puc/v4/Vcs/Api.php
index ecfafff..7d5dd32 100644
--- a/Puc/v4/VcsApi.php
+++ b/Puc/v4/Vcs/Api.php
@@ -1,9 +1,46 @@
 <?php
-if ( !class_exists('Puc_v4_VcsApi') ):
+if ( !class_exists('Puc_v4_Vcs_Api') ):
 
-	abstract class Puc_v4_VcsApi {
+	abstract class Puc_v4_Vcs_Api {
 		protected $tagNameProperty = 'name';
 
+		/**
+		 * @var string
+		 */
+		protected $repositoryUrl = '';
+
+		/**
+		 * @var mixed Authentication details for private repositories. Format depends on service.
+		 */
+		protected $credentials = null;
+
+		/**
+		 * Puc_v4_Vcs_Api constructor.
+		 *
+		 * @param string $repositoryUrl
+		 * @param array|string|null $credentials
+		 */
+		public function __construct($repositoryUrl, $credentials = null) {
+			$this->repositoryUrl = $repositoryUrl;
+			$this->setAuthentication($credentials);
+		}
+
+		/**
+		 * @return string
+		 */
+		public function getRepositoryUrl() {
+			return $this->repositoryUrl;
+		}
+
+		/**
+		 * Figure out which reference (i.e tag or branch) contains the latest version.
+		 *
+		 * @param string $configBranch Start looking in this branch.
+		 * @param bool $useStableTag
+		 * @return null|Puc_v4_Vcs_Reference
+		 */
+		abstract public function chooseReference($configBranch, $useStableTag = true);
+
 		/**
 		 * Get the readme.txt file from the remote repository and parse it
 		 * according to the plugin readme standard.
@@ -25,7 +62,7 @@ if ( !class_exists('Puc_v4_VcsApi') ):
 		 * Get a branch.
 		 *
 		 * @param string $branchName
-		 * @return Puc_v4_VcsReference|null
+		 * @return Puc_v4_Vcs_Reference|null
 		 */
 		abstract public function getBranch($branchName);
 
@@ -33,7 +70,7 @@ if ( !class_exists('Puc_v4_VcsApi') ):
 		 * Get a specific tag.
 		 *
 		 * @param string $tagName
-		 * @return Puc_v4_VcsReference|null
+		 * @return Puc_v4_Vcs_Reference|null
 		 */
 		abstract public function getTag($tagName);
 
@@ -41,7 +78,7 @@ if ( !class_exists('Puc_v4_VcsApi') ):
 		 * Get the tag that looks like the highest version number.
 		 * (Implementations should skip pre-release versions if possible.)
 		 *
-		 * @return Puc_v4_VcsReference|null
+		 * @return Puc_v4_Vcs_Reference|null
 		 */
 		abstract public function getLatestTag();
 
@@ -64,6 +101,33 @@ if ( !class_exists('Puc_v4_VcsApi') ):
 			return (preg_match('@^(\d{1,5}?)(\.\d{1,10}?){0,4}?($|[abrdp+_\-]|\s)@i', $name) === 1);
 		}
 
+		/**
+		 * Check if a tag appears to be named like a version number.
+		 *
+		 * @param stdClass $tag
+		 * @return bool
+		 */
+		protected function isVersionTag($tag) {
+			$property = $this->tagNameProperty;
+			return isset($tag->$property) && $this->looksLikeVersion($tag->$property);
+		}
+
+		/**
+		 * Sort a list of tags as if they were version numbers.
+		 * Tags that don't look like version number will be removed.
+		 *
+		 * @param stdClass[] $tags Array of tag objects.
+		 * @return stdClass[] Filtered array of tags sorted in descending order.
+		 */
+		protected function sortTagsByVersion($tags) {
+			//Keep only those tags that look like version numbers.
+			$versionTags = array_filter($tags, array($this, 'isVersionTag'));
+			//Sort them in descending order.
+			usort($versionTags, array($this, 'compareTagNames'));
+
+			return $versionTags;
+		}
+
 		/**
 		 * Compare two tags as if they were version number.
 		 *
@@ -141,6 +205,23 @@ if ( !class_exists('Puc_v4_VcsApi') ):
 			}
 			return null;
 		}
+
+		/**
+		 * Set authentication credentials.
+		 *
+		 * @param $credentials
+		 */
+		public function setAuthentication($credentials) {
+			$this->credentials = $credentials;
+		}
+
+		/**
+		 * @param string $url
+		 * @return string
+		 */
+		public function signDownloadUrl($url) {
+			return $url;
+		}
 	}
 
 endif;
diff --git a/Puc/v4/Vcs/BaseChecker.php b/Puc/v4/Vcs/BaseChecker.php
new file mode 100644
index 0000000..77a8919
--- /dev/null
+++ b/Puc/v4/Vcs/BaseChecker.php
@@ -0,0 +1,22 @@
+<?php
+if ( !interface_exists('Puc_v4_Vcs_BaseChecker', false) ):
+
+	interface Puc_v4_Vcs_BaseChecker {
+		/**
+		 * Set the repository branch to use for updates. Defaults to 'master'.
+		 *
+		 * @param string $branch
+		 * @return $this
+		 */
+		public function setBranch($branch);
+
+		/**
+		 * Set authentication credentials.
+		 *
+		 * @param array|string $credentials
+		 * @return $this
+		 */
+		public function setAuthentication($credentials);
+	}
+
+endif;
\ No newline at end of file
diff --git a/Puc/v4/BitBucket/Api.php b/Puc/v4/Vcs/BitBucketApi.php
similarity index 65%
rename from Puc/v4/BitBucket/Api.php
rename to Puc/v4/Vcs/BitBucketApi.php
index c7c4a46..a82139f 100644
--- a/Puc/v4/BitBucket/Api.php
+++ b/Puc/v4/Vcs/BitBucketApi.php
@@ -1,7 +1,7 @@
 <?php
-if ( !class_exists('Puc_v4_BitBucket_Api', false) ):
+if ( !class_exists('Puc_v4_Vcs_BitBucketApi', false) ):
 
-	class Puc_v4_BitBucket_Api extends Puc_v4_VcsApi {
+	class Puc_v4_Vcs_BitBucketApi extends Puc_v4_Vcs_Api {
 		/**
 		 * @var Puc_v4_OAuthSignature
 		 */
@@ -12,19 +12,12 @@ if ( !class_exists('Puc_v4_BitBucket_Api', false) ):
 		 */
 		private $username;
 
-		/**
-		 * @var string
-		 */
-		private $repositoryUrl;
-
 		/**
 		 * @var string
 		 */
 		private $repository;
 
 		public function __construct($repositoryUrl, $credentials = array()) {
-			$this->repositoryUrl = $repositoryUrl;
-
 			$path = @parse_url($repositoryUrl, PHP_URL_PATH);
 			if ( preg_match('@^/?(?P<username>[^/]+?)/(?P<repository>[^/#?&]+?)/?$@', $path, $matches) ) {
 				$this->username = $matches['username'];
@@ -33,12 +26,45 @@ if ( !class_exists('Puc_v4_BitBucket_Api', false) ):
 				throw new InvalidArgumentException('Invalid BitBucket repository URL: "' . $repositoryUrl . '"');
 			}
 
-			if ( !empty($credentials) && !empty($credentials['consumer_key']) ) {
-				$this->oauth = new Puc_v4_OAuthSignature(
-					$credentials['consumer_key'],
-					$credentials['consumer_secret']
-				);
+			parent::__construct($repositoryUrl, $credentials);
+		}
+
+		/**
+		 * Figure out which reference (i.e tag or branch) contains the latest version.
+		 *
+		 * @param string $configBranch Start looking in this branch.
+		 * @param bool $useStableTag
+		 * @return null|Puc_v4_Vcs_Reference
+		 */
+		public function chooseReference($configBranch, $useStableTag = true) {
+			$updateSource = null;
+
+			//Check if there's a "Stable tag: 1.2.3" header that points to a valid tag.
+			if ( $useStableTag ) {
+				$remoteReadme = $this->getRemoteReadme($configBranch);
+				if ( !empty($remoteReadme['stable_tag']) ) {
+					$tag = $remoteReadme['stable_tag'];
+
+					//You can explicitly opt out of using tags by setting "Stable tag" to
+					//"trunk" or the name of the current branch.
+					if ( ($tag === $configBranch) || ($tag === 'trunk') ) {
+						return $this->getBranch($configBranch);
+					}
+
+					$updateSource = $this->getTag($tag);
+				}
 			}
+
+			//Look for version-like tags.
+			if ( !$updateSource && ($configBranch === 'master') ) {
+				$updateSource = $this->getLatestTag();
+			}
+			//If all else fails, use the specified branch itself.
+			if ( !$updateSource ) {
+				$updateSource = $this->getBranch($configBranch);
+			}
+
+			return $updateSource;
 		}
 
 		public function getBranch($branchName) {
@@ -47,7 +73,7 @@ if ( !class_exists('Puc_v4_BitBucket_Api', false) ):
 				return null;
 			}
 
-			return new Puc_v4_VcsReference(array(
+			return new Puc_v4_Vcs_Reference(array(
 				'name' => $branch->name,
 				'updated' => $branch->target->date,
 				'downloadUrl' => $this->getDownloadUrl($branch->name),
@@ -58,7 +84,7 @@ if ( !class_exists('Puc_v4_BitBucket_Api', false) ):
 		 * Get a specific tag.
 		 *
 		 * @param string $tagName
-		 * @return Puc_v4_VcsReference|null
+		 * @return Puc_v4_Vcs_Reference|null
 		 */
 		public function getTag($tagName) {
 			$tag = $this->api('/refs/tags/' . $tagName);
@@ -66,7 +92,7 @@ if ( !class_exists('Puc_v4_BitBucket_Api', false) ):
 				return null;
 			}
 
-			return new Puc_v4_VcsReference(array(
+			return new Puc_v4_Vcs_Reference(array(
 				'name' => $tag->name,
 				'version' => ltrim($tag->name, 'v'),
 				'updated' => $tag->target->date,
@@ -77,7 +103,7 @@ if ( !class_exists('Puc_v4_BitBucket_Api', false) ):
 		/**
 		 * Get the tag that looks like the highest version number.
 		 *
-		 * @return Puc_v4_VcsReference|null
+		 * @return Puc_v4_Vcs_Reference|null
 		 */
 		public function getLatestTag() {
 			$tags = $this->api('/refs/tags');
@@ -85,15 +111,13 @@ if ( !class_exists('Puc_v4_BitBucket_Api', false) ):
 				return null;
 			}
 
-			//Keep only those tags that look like version numbers.
-			$versionTags = array_filter($tags->values, array($this, 'isVersionTag'));
-			//Sort them in descending order.
-			usort($versionTags, array($this, 'compareTagNames'));
+			//Filter and sort the list of tags.
+			$versionTags = $this->sortTagsByVersion($tags->values);
 
 			//Return the first result.
 			if ( !empty($versionTags) ) {
 				$tag = $versionTags[0];
-				return new Puc_v4_VcsReference(array(
+				return new Puc_v4_Vcs_Reference(array(
 					'name' => $tag->name,
 					'version' => ltrim($tag->name, 'v'),
 					'updated' => $tag->target->date,
@@ -111,10 +135,6 @@ if ( !class_exists('Puc_v4_BitBucket_Api', false) ):
 			return trailingslashit($this->repositoryUrl) . 'get/' . $ref . '.zip';
 		}
 
-		protected function isVersionTag($tag) {
-			return isset($tag->name) && $this->looksLikeVersion($tag->name);
-		}
-
 		/**
 		 * Get the contents of a file from a specific branch or tag.
 		 *
@@ -185,6 +205,32 @@ if ( !class_exists('Puc_v4_BitBucket_Api', false) ):
 				'BitBucket API error. HTTP status: ' . $code
 			);
 		}
+
+		/**
+		 * @param array $credentials
+		 */
+		public function setAuthentication($credentials) {
+			parent::setAuthentication($credentials);
+
+			if ( !empty($credentials) && !empty($credentials['consumer_key']) ) {
+				$this->oauth = new Puc_v4_OAuthSignature(
+					$credentials['consumer_key'],
+					$credentials['consumer_secret']
+				);
+			} else {
+				$this->oauth = null;
+			}
+		}
+
+		public function signDownloadUrl($url) {
+			//Add authentication data to download URLs. Since OAuth signatures incorporate
+			//timestamps, we have to do this immediately before inserting the update. Otherwise
+			//authentication could fail due to a stale timestamp.
+			if ( $this->oauth ) {
+				$url = $this->oauth->sign($url);
+			}
+			return $url;
+		}
 	}
 
 endif;
\ No newline at end of file
diff --git a/Puc/v4/GitHub/Api.php b/Puc/v4/Vcs/GitHubApi.php
similarity index 75%
rename from Puc/v4/GitHub/Api.php
rename to Puc/v4/Vcs/GitHubApi.php
index 2dc3558..3e3e74d 100644
--- a/Puc/v4/GitHub/Api.php
+++ b/Puc/v4/Vcs/GitHubApi.php
@@ -1,8 +1,8 @@
 <?php
 
-if ( !class_exists('Puc_v4_GitHub_Api', false) ):
+if ( !class_exists('Puc_v4_Vcs_GitHubApi', false) ):
 
-	class Puc_v4_GitHub_Api extends Puc_v4_VcsApi {
+	class Puc_v4_Vcs_GitHubApi extends Puc_v4_Vcs_Api {
 		/**
 		 * @var string GitHub username.
 		 */
@@ -23,9 +23,6 @@ if ( !class_exists('Puc_v4_GitHub_Api', false) ):
 		protected $accessToken;
 
 		public function __construct($repositoryUrl, $accessToken = null) {
-			$this->repositoryUrl = $repositoryUrl;
-			$this->accessToken = $accessToken;
-
 			$path = @parse_url($repositoryUrl, PHP_URL_PATH);
 			if ( preg_match('@^/?(?P<username>[^/]+?)/(?P<repository>[^/#?&]+?)/?$@', $path, $matches) ) {
 				$this->userName = $matches['username'];
@@ -33,12 +30,14 @@ if ( !class_exists('Puc_v4_GitHub_Api', false) ):
 			} else {
 				throw new InvalidArgumentException('Invalid GitHub repository URL: "' . $repositoryUrl . '"');
 			}
+
+			parent::__construct($repositoryUrl, $accessToken);
 		}
 
 		/**
 		 * Get the latest release from GitHub.
 		 *
-		 * @return Puc_v4_VcsReference|null
+		 * @return Puc_v4_Vcs_Reference|null
 		 */
 		public function getLatestRelease() {
 			$release = $this->api('/repos/:user/:repo/releases/latest');
@@ -46,10 +45,10 @@ if ( !class_exists('Puc_v4_GitHub_Api', false) ):
 				return null;
 			}
 
-			$reference = new Puc_v4_VcsReference(array(
+			$reference = new Puc_v4_Vcs_Reference(array(
 				'name' => $release->tag_name,
 				'version' => ltrim($release->tag_name, 'v'), //Remove the "v" prefix from "v1.2.3".
-				'downloadUrl' => $release->zipball_url,
+				'downloadUrl' => $this->signDownloadUrl($release->zipball_url),
 				'updated' => $release->created_at,
 			));
 
@@ -67,7 +66,7 @@ if ( !class_exists('Puc_v4_GitHub_Api', false) ):
 		/**
 		 * Get the tag that looks like the highest version number.
 		 *
-		 * @return Puc_v4_VcsReference|null
+		 * @return Puc_v4_Vcs_Reference|null
 		 */
 		public function getLatestTag() {
 			$tags = $this->api('/repos/:user/:repo/tags');
@@ -76,13 +75,16 @@ if ( !class_exists('Puc_v4_GitHub_Api', false) ):
 				return null;
 			}
 
-			usort($tags, array($this, 'compareTagNames')); //Sort from highest to lowest.
+			$versionTags = $this->sortTagsByVersion($tags);
+			if ( empty($versionTags) ) {
+				return null;
+			}
 
-			$tag = $tags[0];
-			return new Puc_v4_VcsReference(array(
+			$tag = $versionTags[0];
+			return new Puc_v4_Vcs_Reference(array(
 				'name' => $tag->name,
 				'version' => ltrim($tag->name, 'v'),
-				'downloadUrl' => $tag->zipball_url,
+				'downloadUrl' => $this->signDownloadUrl($tag->zipball_url),
 			));
 		}
 
@@ -90,7 +92,7 @@ if ( !class_exists('Puc_v4_GitHub_Api', false) ):
 		 * Get a branch by name.
 		 *
 		 * @param string $branchName
-		 * @return null|Puc_v4_VcsReference
+		 * @return null|Puc_v4_Vcs_Reference
 		 */
 		public function getBranch($branchName) {
 			$branch = $this->api('/repos/:user/:repo/branches/' . $branchName);
@@ -98,7 +100,7 @@ if ( !class_exists('Puc_v4_GitHub_Api', false) ):
 				return null;
 			}
 
-			$reference = new Puc_v4_VcsReference(array(
+			$reference = new Puc_v4_Vcs_Reference(array(
 				'name' => $branch->name,
 				'downloadUrl' => $this->buildArchiveDownloadUrl($branch->name),
 			));
@@ -218,7 +220,7 @@ if ( !class_exists('Puc_v4_GitHub_Api', false) ):
 				urlencode($ref)
 			);
 			if ( !empty($this->accessToken) ) {
-				$url = add_query_arg('access_token', $this->accessToken, $url);
+				$url = $this->signDownloadUrl($url);
 			}
 			return $url;
 		}
@@ -227,12 +229,43 @@ if ( !class_exists('Puc_v4_GitHub_Api', false) ):
 		 * Get a specific tag.
 		 *
 		 * @param string $tagName
-		 * @return Puc_v4_VcsReference|null
+		 * @return Puc_v4_Vcs_Reference|null
 		 */
 		public function getTag($tagName) {
-			//The current GitHub update checker doesn't use getTag, so didn't bother to implement it.
+			//The current GitHub update checker doesn't use getTag, so I didn't bother to implement it.
 			throw new LogicException('The ' . __METHOD__ . ' method is not implemented and should not be used.');
 		}
+
+		public function setAuthentication($credentials) {
+			parent::setAuthentication($credentials);
+			$this->accessToken = is_string($credentials) ? $credentials : null;
+		}
+
+		/**
+		 * Figure out which reference (i.e tag or branch) contains the latest version.
+		 *
+		 * @param string $configBranch Start looking in this branch.
+		 * @param bool $useStableTag Ignored. The GitHub client doesn't use the "Stable tag" header.
+		 * @return null|Puc_v4_Vcs_Reference
+		 */
+		public function chooseReference($configBranch, $useStableTag = false) {
+			$updateSource = null;
+
+			if ( $configBranch === 'master' ) {
+				//Use the latest release.
+				$updateSource = $this->getLatestRelease();
+				if ( $updateSource === null ) {
+					//Failing that, use the tag with the highest version number.
+					$updateSource = $this->getLatestTag();
+				}
+			}
+			//Alternatively, just use the branch itself.
+			if ( empty($updateSource) ) {
+				$updateSource = $this->getBranch($configBranch);
+			}
+
+			return $updateSource;
+		}
 	}
 
 endif;
\ No newline at end of file
diff --git a/Puc/v4/BitBucket/PluginUpdateChecker.php b/Puc/v4/Vcs/PluginUpdateChecker.php
similarity index 68%
rename from Puc/v4/BitBucket/PluginUpdateChecker.php
rename to Puc/v4/Vcs/PluginUpdateChecker.php
index a3dae8c..da2ba5f 100644
--- a/Puc/v4/BitBucket/PluginUpdateChecker.php
+++ b/Puc/v4/Vcs/PluginUpdateChecker.php
@@ -1,25 +1,38 @@
 <?php
-if ( !class_exists('Puc_v4_BitBucket_PluginUpdateChecker') ):
+if ( !class_exists('Puc_v4_Vcs_PluginUpdateChecker') ):
 
-	class Puc_v4_BitBucket_PluginUpdateChecker extends Puc_v4_Plugin_UpdateChecker {
+	class Puc_v4_Vcs_PluginUpdateChecker extends Puc_v4_Plugin_UpdateChecker implements Puc_v4_Vcs_BaseChecker {
 		/**
-		 * @var string
+		 * @var string The branch where to look for updates. Defaults to "master".
 		 */
 		protected $branch = 'master';
 
 		/**
-		 * @var Puc_v4_BitBucket_Api
+		 * @var Puc_v4_Vcs_Api Repository API client.
 		 */
-		protected $api;
+		protected $api = null;
 
-		protected $credentials = array();
+		/**
+		 * Puc_v4_Vcs_PluginUpdateChecker constructor.
+		 *
+		 * @param Puc_v4_Vcs_Api $api
+		 * @param string $pluginFile
+		 * @param string $slug
+		 * @param int $checkPeriod
+		 * @param string $optionName
+		 * @param string $muPluginFile
+		 */
+		public function __construct($api, $pluginFile, $slug = '', $checkPeriod = 12, $optionName = '', $muPluginFile = '') {
+			$this->api = $api;
+			parent::__construct($api->getRepositoryUrl(), $pluginFile, $slug, $checkPeriod, $optionName, $muPluginFile);
+		}
 
 		public function requestInfo($queryArgs = array()) {
 			//We have to make several remote API requests to gather all the necessary info
 			//which can take a while on slow networks.
 			set_time_limit(60);
 
-			$api = $this->api = new Puc_v4_BitBucket_Api($this->metadataUrl, $this->credentials);
+			$api = $this->api;
 
 			$info = new Puc_v4_Plugin_Info();
 			$info->filename = $this->pluginFile;
@@ -28,12 +41,19 @@ if ( !class_exists('Puc_v4_BitBucket_PluginUpdateChecker') ):
 			$this->setInfoFromHeader($this->getPluginHeader(), $info);
 
 			//Pick a branch or tag.
-			$updateSource = $this->chooseReference();
+			$updateSource = $api->chooseReference($this->branch);
 			if ( $updateSource ) {
 				$ref = $updateSource->name;
 				$info->version = $updateSource->version;
 				$info->last_updated = $updateSource->updated;
 				$info->download_url = $updateSource->downloadUrl;
+
+				if ( !empty($updateSource->changelog) ) {
+					$info->sections['changelog'] = $updateSource->changelog;
+				}
+				if ( isset($updateSource->downloadCount) ) {
+					$info->downloaded = $updateSource->downloadCount;
+				}
 			} else {
 				//There's probably a network problem or an authentication error.
 				return null;
@@ -74,56 +94,6 @@ if ( !class_exists('Puc_v4_BitBucket_PluginUpdateChecker') ):
 			return $info;
 		}
 
-		/**
-		 * Figure out which reference (tag or branch) we'll use to get the latest version of the plugin.
-		 *
-		 * @return Puc_v4_VcsReference|null
-		 */
-		protected function chooseReference() {
-			$api = $this->api;
-			$updateSource = null;
-
-			//Check if there's a "Stable tag: v1.2.3" header that points to a valid tag.
-			$remoteReadme = $api->getRemoteReadme($this->branch);
-			if ( !empty($remoteReadme['stable_tag']) ) {
-				$tag = $remoteReadme['stable_tag'];
-
-				//You can explicitly opt out of using tags by setting "Stable tag" to
-				//"trunk" or the name of the current branch.
-				if ( ($tag === $this->branch) || ($tag === 'trunk') ) {
-					return $api->getBranch($this->branch);
-				}
-
-				$updateSource = $api->getTag($tag);
-			}
-			//Look for version-like tags.
-			if ( !$updateSource && ($this->branch === 'master') ) {
-				$updateSource = $api->getLatestTag();
-			}
-			//If all else fails, use the specified branch itself.
-			if ( !$updateSource ) {
-				$updateSource = $api->getBranch($this->branch);
-			}
-
-			return $updateSource;
-		}
-
-		public function setAuthentication($credentials) {
-			$this->credentials = array_merge(
-				array(
-					'consumer_key' => '',
-					'consumer_secret' => '',
-				),
-				$credentials
-			);
-			return $this;
-		}
-
-		public function setBranch($branchName = 'master') {
-			$this->branch = $branchName;
-			return $this;
-		}
-
 		/**
 		 * Check if the currently installed version has a readme.txt file.
 		 *
@@ -195,20 +165,21 @@ if ( !class_exists('Puc_v4_BitBucket_PluginUpdateChecker') ):
 			}
 		}
 
+		public function setBranch($branch) {
+			$this->branch = $branch;
+			return $this;
+		}
+
+		public function setAuthentication($credentials) {
+			$this->api->setAuthentication($credentials);
+			return $this;
+		}
+
 		public function getUpdate() {
 			$update = parent::getUpdate();
 
-			//Add authentication data to download URLs. Since OAuth signatures incorporate
-			//timestamps, we have to do this immediately before inserting the update. Otherwise
-			//authentication could fail due to a stale timestamp.
-			if ( isset($update, $update->download_url) && !empty($update->download_url) && !empty($this->credentials) ) {
-				if ( !empty($this->credentials['consumer_key']) ) {
-					$oauth = new Puc_v4_OAuthSignature(
-						$this->credentials['consumer_key'],
-						$this->credentials['consumer_secret']
-					);
-					$update->download_url = $oauth->sign($update->download_url);
-				}
+			if ( isset($update) && !empty($update->download_url) ) {
+				$update->download_url = $this->api->signDownloadUrl($update->download_url);
 			}
 
 			return $update;
diff --git a/Puc/v4/VcsReference.php b/Puc/v4/Vcs/Reference.php
similarity index 91%
rename from Puc/v4/VcsReference.php
rename to Puc/v4/Vcs/Reference.php
index e7047fa..501955f 100644
--- a/Puc/v4/VcsReference.php
+++ b/Puc/v4/Vcs/Reference.php
@@ -1,5 +1,5 @@
 <?php
-if ( !class_exists('Puc_v4_VcsReference', false) ):
+if ( !class_exists('Puc_v4_Vcs_Reference', false) ):
 
 	/**
 	 * This class represents a VCS branch or tag. It's intended as a read only, short-lived container
@@ -13,7 +13,7 @@ if ( !class_exists('Puc_v4_VcsReference', false) ):
 	 * @property string|null $changelog
 	 * @property int|null $downloadCount
 	 */
-	class Puc_v4_VcsReference {
+	class Puc_v4_Vcs_Reference {
 		private $properties = array();
 
 		public function __construct($properties = array()) {
diff --git a/Puc/v4/Vcs/ThemeUpdateChecker.php b/Puc/v4/Vcs/ThemeUpdateChecker.php
new file mode 100644
index 0000000..5390b3d
--- /dev/null
+++ b/Puc/v4/Vcs/ThemeUpdateChecker.php
@@ -0,0 +1,99 @@
+<?php
+
+if ( !class_exists('Puc_v4_Vcs_ThemeUpdateChecker', false) ):
+
+	class Puc_v4_Vcs_ThemeUpdateChecker extends Puc_v4_Theme_UpdateChecker implements Puc_v4_Vcs_BaseChecker {
+		/**
+		 * @var string The branch where to look for updates. Defaults to "master".
+		 */
+		protected $branch = 'master';
+
+		/**
+		 * @var Puc_v4_Vcs_Api Repository API client.
+		 */
+		protected $api = null;
+
+		/**
+		 * Puc_v4_Vcs_ThemeUpdateChecker constructor.
+		 *
+		 * @param Puc_v4_Vcs_Api $api
+		 * @param null $stylesheet
+		 * @param null $customSlug
+		 * @param int $checkPeriod
+		 * @param string $optionName
+		 */
+		public function __construct($api, $stylesheet = null, $customSlug = null, $checkPeriod = 12, $optionName = '') {
+			$this->api = $api;
+			parent::__construct($api->getRepositoryUrl(), $stylesheet, $customSlug, $checkPeriod, $optionName);
+		}
+
+		public function requestUpdate() {
+			$api = $this->api;
+
+			$update = new Puc_v4_Theme_Update();
+			$update->slug = $this->slug;
+
+			//Figure out which reference (tag or branch) we'll use to get the latest version of the theme.
+			$updateSource = $api->chooseReference($this->branch, false);
+			if ( $updateSource ) {
+				$ref = $updateSource->name;
+				$update->version = $updateSource->version;
+				$update->download_url = $updateSource->downloadUrl;
+			} else {
+				$ref = $this->branch;
+			}
+
+			//Get headers from the main stylesheet in this branch/tag. Its "Version" header and other metadata
+			//are what the WordPress install will actually see after upgrading, so they take precedence over releases/tags.
+			$remoteStylesheet = $api->getRemoteFile('style.css', $ref);
+			if ( !empty($remoteStylesheet) ) {
+				$remoteHeader = $this->getFileHeader($remoteStylesheet);
+				if ( !empty($remoteHeader['Version']) ) {
+					$update->version = $remoteHeader['Version'];
+				}
+				if ( !empty($remoteHeader['ThemeURI']) ) {
+					$update->details_url = $remoteHeader['ThemeURI'];
+				}
+			}
+
+			//The details URL defaults to the Theme URI header or the repository URL.
+			if ( empty($update->details_url) ) {
+				$update->details_url = $this->theme->get('ThemeURI');
+			}
+			if ( empty($update->details_url) ) {
+				$update->details_url = $this->metadataUrl;
+			}
+
+			if ( empty($update->version) ) {
+				//It looks like we didn't find a valid update after all.
+				$update = null;
+			}
+
+			$update = $this->filterUpdateResult($update);
+			return $update;
+		}
+
+		public function setBranch($branch) {
+			$this->branch = $branch;
+			return $this;
+		}
+
+		public function setAuthentication($credentials) {
+			$this->api->setAuthentication($credentials);
+			return $this;
+		}
+
+		public function getUpdate() {
+			$update = parent::getUpdate();
+
+			if ( isset($update) && !empty($update->download_url) ) {
+				$update->download_url = $this->api->signDownloadUrl($update->download_url);
+			}
+
+			return $update;
+		}
+
+
+	}
+
+endif;
\ No newline at end of file
diff --git a/plugin-update-checker.php b/plugin-update-checker.php
index 81dbd87..642809a 100644
--- a/plugin-update-checker.php
+++ b/plugin-update-checker.php
@@ -13,7 +13,9 @@ new Puc_v4_Autoloader();
 //Register classes defined in this file with the factory.
 Puc_v4_Factory::addVersion('Plugin_UpdateChecker', 'Puc_v4_Plugin_UpdateChecker', '4.0');
 Puc_v4_Factory::addVersion('Theme_UpdateChecker', 'Puc_v4_Theme_UpdateChecker', '4.0');
-Puc_v4_Factory::addVersion('GitHub_PluginUpdateChecker', 'Puc_v4_GitHub_PluginUpdateChecker', '4.0');
-Puc_v4_Factory::addVersion('GitHub_ThemeUpdateChecker', 'Puc_v4_GitHub_ThemeUpdateChecker', '4.0');
 
-Puc_v4_Factory::addVersion('BitBucket_PluginUpdateChecker', 'Puc_v4_BitBucket_PluginUpdateChecker', '4.0');
+Puc_v4_Factory::addVersion('Vcs_PluginUpdateChecker', 'Puc_v4_Vcs_PluginUpdateChecker', '4.0');
+Puc_v4_Factory::addVersion('Vcs_ThemeUpdateChecker', 'Puc_v4_Vcs_ThemeUpdateChecker', '4.0');
+
+Puc_v4_Factory::addVersion('GitHubApi', 'Puc_v4_Vcs_GitHubApi', '4.0');
+Puc_v4_Factory::addVersion('BitBucketApi', 'Puc_v4_Vcs_BitBucketApi', '4.0');