diff --git a/wp-includes/bookmark-template.php b/wp-includes/bookmark-template.php
index 2a06cf2a2a..3d644a6be9 100644
--- a/wp-includes/bookmark-template.php
+++ b/wp-includes/bookmark-template.php
@@ -167,7 +167,7 @@ function get_linkcatname($id = 0) {
 	$cat_id = (int) $cats[0]; // Take the first cat.
 
 	$cat = get_category($cat_id);
-	return $cat->cat_name;
+	return $cat->name;
 }
 
 /** function links_popup_script()
@@ -229,9 +229,9 @@ function get_links_list($order = 'name', $hide_if_empty = 'obsolete') {
 			// Handle each category.
 
 			// Display the category name
-			echo '	<li id="linkcat-' . $cat->cat_ID . '" class="linkcat"><h2>' . $cat->cat_name . "</h2>\n\t<ul>\n";
+			echo '	<li id="linkcat-' . $cat->term_id . '" class="linkcat"><h2>' . $cat->name . "</h2>\n\t<ul>\n";
 			// Call get_links() with all the appropriate params
-			get_links($cat->cat_ID, '<li>', "</li>", "\n", true, 'name', false);
+			get_links($cat->term_id, '<li>', "</li>", "\n", true, 'name', false);
 
 			// Close the last category
 			echo "\n\t</ul>\n</li>\n";
@@ -336,12 +336,12 @@ function wp_list_bookmarks($args = '') {
 		$cats = get_categories("type=link&category_name=$category_name&include=$category&orderby=$category_orderby&order=$category_order&hierarchical=0");
 
 		foreach ( (array) $cats as $cat ) {
-			$params = array_merge($r, array('category'=>$cat->cat_ID));
+			$params = array_merge($r, array('category'=>$cat->term_id));
 			$bookmarks = get_bookmarks($params);
 			if ( empty($bookmarks) )
 				continue;
-			$output .= str_replace(array('%id', '%class'), array("linkcat-$cat->cat_ID", $class), $category_before);
-			$catname = apply_filters( "link_category", $cat->cat_name );
+			$output .= str_replace(array('%id', '%class'), array("linkcat-$cat->term_id", $class), $category_before);
+			$catname = apply_filters( "link_category", $cat->name );
 			$output .= "$title_before$catname$title_after\n\t<ul>\n";
 			$output .= _walk_bookmarks($bookmarks, $r);
 			$output .= "\n\t</ul>\n$category_after\n";
diff --git a/wp-includes/category-template.php b/wp-includes/category-template.php
index 78a6e34f10..6aa27db6b7 100644
--- a/wp-includes/category-template.php
+++ b/wp-includes/category-template.php
@@ -5,10 +5,10 @@ function get_category_children($id, $before = '/', $after = '') {
 		return '';
 
 	$chain = '';
-
+	// TODO: consult hierarchy
 	$cat_ids = get_all_category_ids();
 	foreach ( $cat_ids as $cat_id ) {
-		if ( $cat_id == $id)
+		if ( $cat_id == $id )
 			continue;
 
 		$category = get_category($cat_id);
diff --git a/wp-includes/classes.php b/wp-includes/classes.php
index 2363803db3..0a285de5b5 100644
--- a/wp-includes/classes.php
+++ b/wp-includes/classes.php
@@ -557,7 +557,7 @@ class Walker_PageDropdown extends Walker {
 
 class Walker_Category extends Walker {
 	var $tree_type = 'category';
-	var $db_fields = array ('parent' => 'category_parent', 'id' => 'cat_ID'); //TODO: decouple this
+	var $db_fields = array ('parent' => 'parent', 'id' => 'term_id'); //TODO: decouple this
 
 	function start_lvl($output, $depth, $args) {
 		if ( 'list' != $args['style'] )
@@ -580,13 +580,13 @@ class Walker_Category extends Walker {
 	function start_el($output, $category, $depth, $args) {
 		extract($args);
 
-		$cat_name = attribute_escape( $category->cat_name);
+		$cat_name = attribute_escape( $category->name);
 		$cat_name = apply_filters( 'list_cats', $cat_name, $category );
-		$link = '<a href="' . get_category_link( $category->cat_ID ) . '" ';
-		if ( $use_desc_for_title == 0 || empty($category->category_description) )
+		$link = '<a href="' . get_category_link( $category->term_id ) . '" ';
+		if ( $use_desc_for_title == 0 || empty($category->description) )
 			$link .= 'title="' . sprintf(__( 'View all posts filed under %s' ), $cat_name) . '"';
 		else
-			$link .= 'title="' . attribute_escape( apply_filters( 'category_description', $category->category_description, $category )) . '"';
+			$link .= 'title="' . attribute_escape( apply_filters( 'category_description', $category->description, $category )) . '"';
 		$link .= '>';
 		$link .= $cat_name . '</a>';
 
@@ -596,7 +596,7 @@ class Walker_Category extends Walker {
 			if ( empty($feed_image) )
 				$link .= '(';
 
-			$link .= '<a href="' . get_category_rss_link( 0, $category->cat_ID, $category->category_nicename ) . '"';
+			$link .= '<a href="' . get_category_rss_link( 0, $category->term_id, $category->slug ) . '"';
 
 			if ( empty($feed) )
 				$alt = ' alt="' . sprintf(__( 'Feed for all posts filed under %s' ), $cat_name ) . '"';
@@ -619,7 +619,7 @@ class Walker_Category extends Walker {
 		}
 
 		if ( isset($show_count) && $show_count )
-			$link .= ' (' . intval($category->category_count) . ')';
+			$link .= ' (' . intval($category->count) . ')';
 
 		if ( isset($show_date) && $show_date ) {
 			$link .= ' ' . gmdate('Y-m-d', $category->last_update_timestamp);
@@ -630,9 +630,9 @@ class Walker_Category extends Walker {
 
 		if ( 'list' == $args['style'] ) {
 			$output .= "\t<li";
-			if ( $current_category && ($category->cat_ID == $current_category) )
+			if ( $current_category && ($category->term_id == $current_category) )
 				$output .=  ' class="current-cat"';
-			elseif ( $_current_category && ($category->cat_ID == $_current_category->category_parent) )
+			elseif ( $_current_category && ($category->term_id == $_current_category->parent) )
 				$output .=  ' class="current-cat-parent"';
 			$output .= ">$link\n";
 		} else {
@@ -654,19 +654,19 @@ class Walker_Category extends Walker {
 
 class Walker_CategoryDropdown extends Walker {
 	var $tree_type = 'category';
-	var $db_fields = array ('parent' => 'category_parent', 'id' => 'cat_ID'); //TODO: decouple this
+	var $db_fields = array ('parent' => 'parent', 'id' => 'term_id'); //TODO: decouple this
 
 	function start_el($output, $category, $depth, $args) {
 		$pad = str_repeat('&nbsp;', $depth * 3);
 
-		$cat_name = apply_filters('list_cats', $category->cat_name, $category);
-		$output .= "\t<option value=\"".$category->cat_ID."\"";
-		if ( $category->cat_ID == $args['selected'] )
+		$cat_name = apply_filters('list_cats', $category->name, $category);
+		$output .= "\t<option value=\"".$category->term_id."\"";
+		if ( $category->term_id == $args['selected'] )
 			$output .= ' selected="selected"';
 		$output .= '>';
 		$output .= $pad.$cat_name;
 		if ( $args['show_count'] )
-			$output .= '&nbsp;&nbsp;('. $category->category_count .')';
+			$output .= '&nbsp;&nbsp;('. $category->count .')';
 		if ( $args['show_last_update'] ) {
 			$format = 'Y-m-d';
 			$output .= '&nbsp;&nbsp;' . gmdate($format, $category->last_update_timestamp);
diff --git a/wp-includes/query.php b/wp-includes/query.php
index 1ac1e1ccec..167a1a7814 100644
--- a/wp-includes/query.php
+++ b/wp-includes/query.php
@@ -76,11 +76,11 @@ function is_category ($category = '') {
 
 	$cat_obj = $wp_query->get_queried_object();
 
-	if ( $category == $cat_obj->cat_ID )
+	if ( $category == $cat_obj->term_id )
 		return true;
-	else if ( $category == $cat_obj->cat_name )
+	else if ( $category == $cat_obj->name )
 		return true;
-	elseif ( $category == $cat_obj->category_nicename )
+	elseif ( $category == $cat_obj->slug )
 		return true;
 
 	return false;
@@ -842,7 +842,7 @@ class WP_Query {
 		} else {
 			$q['cat'] = ''.urldecode($q['cat']).'';
 			$q['cat'] = addslashes_gpc($q['cat']);
-			$join = " LEFT JOIN $wpdb->post2cat ON ($wpdb->posts.ID = $wpdb->post2cat.post_id) ";
+			$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) ";
 			$cat_array = preg_split('/[,\s]+/', $q['cat']);
 			$in_cats = $out_cats = $out_posts = '';
 			foreach ( $cat_array as $cat ) {
@@ -857,8 +857,9 @@ class WP_Query {
 			$in_cats = substr($in_cats, 0, -2);
 			$out_cats = substr($out_cats, 0, -2);
 			if ( strlen($in_cats) > 0 )
-				$in_cats = " AND $wpdb->post2cat.category_id IN ($in_cats) AND rel_type = 'category' ";
+				$in_cats = " AND $wpdb->term_taxonomy.term_id IN ({$q['cat']}) ";
 			if ( strlen($out_cats) > 0 ) {
+				// TODO
 				$ids = $wpdb->get_col("SELECT post_id FROM $wpdb->post2cat WHERE $wpdb->post2cat.category_id IN ($out_cats)");
 				if ( is_array($ids) && count($ids > 0) ) {
 					foreach ( $ids as $id )
@@ -870,7 +871,8 @@ class WP_Query {
 				else
 					$out_cats = '';
 			}
-			$whichcat = $in_cats . $out_cats;
+			$whichcat = " AND $wpdb->term_taxonomy.taxonomy = 'category' ";
+			$whichcat .= $in_cats . $out_cats;
 			$groupby = "{$wpdb->posts}.ID";
 		}
 
@@ -882,8 +884,6 @@ class WP_Query {
 				$reqtag = 0;
 
 			$q['tag_id'] = $reqtag;
-			// TODO: use term taxonomy
-			$tables = ", $wpdb->post2cat, $wpdb->categories";
 			$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";
@@ -908,18 +908,18 @@ class WP_Query {
 				$reqcat = get_category_by_path($q['category_name'], false);
 
 			if ( !empty($reqcat) )
-				$reqcat = $reqcat->cat_ID;
+				$reqcat = $reqcat->term_id;
 			else
 				$reqcat = 0;
 
 			$q['cat'] = $reqcat;
 
-			$tables = ", $wpdb->post2cat, $wpdb->categories";
-			$join = " LEFT JOIN $wpdb->post2cat ON ($wpdb->posts.ID = $wpdb->post2cat.post_id) LEFT JOIN $wpdb->categories ON ($wpdb->post2cat.category_id = $wpdb->categories.cat_ID) ";
-			$whichcat = " AND category_id IN ({$q['cat']}, ";
+			$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 = 'category' ";
+			$whichcat .= "AND $wpdb->term_taxonomy.term_id IN ({$q['cat']}, ";
 			$whichcat .= get_category_children($q['cat'], '', ', ');
 			$whichcat = substr($whichcat, 0, -2);
-			$whichcat .= ") AND rel_type = 'category'";
+			$whichcat .= ")";
 			$groupby = "{$wpdb->posts}.ID";
 		}