diff --git a/wp-includes/classes.php b/wp-includes/classes.php
index f1798e3d01..ff1396486d 100644
--- a/wp-includes/classes.php
+++ b/wp-includes/classes.php
@@ -1,9 +1,9 @@
 <?php
 
 class WP {
-	var $public_query_vars = array('m', 'p', 'posts', 'w', 'cat', 'withcomments', 'withoutcomments', 's', 'search', 'exact', 'sentence', 'debug', 'calendar', 'page', 'paged', 'more', 'tb', 'pb', 'author', 'order', 'orderby', 'year', 'monthnum', 'day', 'hour', 'minute', 'second', 'name', 'category_name', 'tag', 'feed', 'author_name', 'static', 'pagename', 'page_id', 'error', 'comments_popup', 'attachment', 'attachment_id', 'subpost', 'subpost_id', 'preview', 'robots');
+	var $public_query_vars = array('m', 'p', 'posts', 'w', 'cat', 'withcomments', 'withoutcomments', 's', 'search', 'exact', 'sentence', 'debug', 'calendar', 'page', 'paged', 'more', 'tb', 'pb', 'author', 'order', 'orderby', 'year', 'monthnum', 'day', 'hour', 'minute', 'second', 'name', 'category_name', 'tag', 'tag_id', 'feed', 'author_name', 'static', 'pagename', 'page_id', 'error', 'comments_popup', 'attachment', 'attachment_id', 'subpost', 'subpost_id', 'preview', 'robots');
 
-	var $private_query_vars = array('offset', 'posts_per_page', 'posts_per_archive_page', 'what_to_show', 'showposts', 'nopaging', 'post_type', 'post_status', 'category__in', 'category__not_in', 'category__and');
+	var $private_query_vars = array('offset', 'posts_per_page', 'posts_per_archive_page', 'what_to_show', 'showposts', 'nopaging', 'post_type', 'post_status', 'category__in', 'category__not_in', 'category__and', 'tag__in', 'tag__not_in', 'tag__and');
 	var $extra_query_vars = array();
 
 	var $query_vars;
diff --git a/wp-includes/query.php b/wp-includes/query.php
index d5334e1486..1b0d6a26f8 100644
--- a/wp-includes/query.php
+++ b/wp-includes/query.php
@@ -405,6 +405,7 @@ class WP_Query {
 			, 'w'
 			, 'category_name'
 			, 'tag'
+			, 'tag_id'
 			, 'author_name'
 			, 'feed'
 			, 'tb'
@@ -418,7 +419,8 @@ class WP_Query {
 				$array[$key] = '';
 		}
 
-		$array_keys = array('category__in', 'category__not_in', 'category__and');
+		$array_keys = array('category__in', 'category__not_in', 'category__and',
+			'tag__in', 'tag__not_in', 'tag__and');
 		
 		foreach ( $array_keys as $key ) {
 			if ( !isset($array[$key]))
@@ -582,6 +584,30 @@ class WP_Query {
 			if (  '' != $qv['tag'] )
 				$this->is_tag = true;
 
+			$qv['tag__id'] = (int) $qv['tag__id'];
+			if (  !empty($qv['tag__id']) )
+				$this->is_tag = true;
+
+			if ( !is_array($qv['tag__in']) || empty($qv['tag__in']) ) {
+				$qv['tag__in'] = array();
+			} else {
+				$qv['tag__in'] = array_map('intval', $qv['tag__in']);
+				$this->is_tag = true;	
+			}
+
+			if ( !is_array($qv['tag___not_in']) || empty($qv['tag__not_in']) ) {
+				$qv['tag__not_in'] = array();
+			} else {
+				$qv['tag__not_in'] = array_map('intval', $qv['tag__not_in']);
+			}
+
+			if ( !is_array($qv['tag__and']) || empty($qv['tag__and']) ) {
+				$qv['tag__and'] = array();
+			} else {
+				$qv['tag__and'] = array_map('intval', $qv['tag__and']);
+				$this->is_category = true;	
+			}
+
 			if ( empty($qv['author']) || ($qv['author'] == '0') ) {
 				$this->is_author = false;
 			} else {
@@ -922,19 +948,6 @@ class WP_Query {
 			}
 		}
 
-		if ( '' != $q['tag'] ) {
-			$reqtag = is_term( $q['tag'], 'post_tag' );
-			if ( !empty($reqtag) )
-				$reqtag = $reqtag['term_id'];
-			else
-				$reqtag = 0;
-
-			$q['tag_id'] = $reqtag;
-			$join = " LEFT JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id) LEFT JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id) ";
-			$whichcat = " AND $wpdb->term_taxonomy.term_id IN ({$q['tag_id']}) AND $wpdb->term_taxonomy.taxonomy = 'post_tag' ";
-			$groupby = "{$wpdb->posts}.ID";
-		}
-
 		// Category stuff for nice URLs
 		if ( '' != $q['category_name'] ) {
 			$reqcat = get_category_by_path($q['category_name']);
@@ -969,6 +982,46 @@ class WP_Query {
 			$groupby = "{$wpdb->posts}.ID";
 		}
 
+		// Tags
+		if ( '' != $q['tag'] ) {
+			$reqtag = is_term( $q['tag'], 'post_tag' );
+			if ( !empty($reqtag) )
+				$reqtag = $reqtag['term_id'];
+			else
+				$reqtag = 0;
+
+			$q['tag_id'] = $reqtag;
+			$q['tag__in'][] = $reqtag;
+		}
+
+		if ( !empty($q['tag__in']) || !empty($q['tag__not_in']) || !empty($q['tag__and']) ) {
+			$groupby = "{$wpdb->posts}.ID";
+		}
+
+		if ( !empty($q['tag__in']) ) {
+			$join = " LEFT JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id) LEFT JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id) ";
+			$whichcat .= " AND $wpdb->term_taxonomy.taxonomy = 'post_tag' ";
+			$include_tags = "'" . implode("', '", $q['tag__in']) . "'";
+			$whichcat .= " AND $wpdb->term_taxonomy.term_id IN ($include_tags) ";
+		}
+
+		if ( !empty($q['tag__not_in']) ) {
+			$ids = get_objects_in_term($q['tag__not_in'], 'post_tag');
+			if ( is_array($ids) && count($ids > 0) ) {
+				$out_posts = "'" . implode("', '", $ids) . "'";
+				$whichcat .= " AND $wpdb->posts.ID NOT IN ($out_posts)";
+			}
+		}
+
+		if ( !empty($q['tag__and']) ) {
+			$count = 0;
+			foreach ( $q['tag__and'] as $tag_and ) {
+				$join .= " LEFT JOIN $wpdb->term_relationships AS tr$count ON ($wpdb->posts.ID = tr$count.object_id) LEFT JOIN $wpdb->term_taxonomy AS tt$count ON (tr$count.term_taxonomy_id = tt$count.term_taxonomy_id) ";
+				$whichcat .= " AND tt$count.term_id = '$tag_and' ";
+				$count++;
+			}
+		}
+
 		// Author/user stuff
 
 		if ( empty($q['author']) || ($q['author'] == '0') ) {