diff --git a/inc/activities/undo.php b/inc/activities/undo.php
new file mode 100644
index 0000000..bc45775
--- /dev/null
+++ b/inc/activities/undo.php
@@ -0,0 +1,201 @@
+<?php
+namespace activities\undo;
+
+require_once plugin_dir_path( __FILE__ ) . '/../util.php';
+require_once plugin_dir_path( __FILE__ ) . '/../activities.php';
+require_once plugin_dir_path( __FILE__ ) . '/../actors.php';
+require_once plugin_dir_path( __FILE__ ) . '/../objects.php';
+require_once plugin_dir_path( __FILE__ ) . '/../likes.php';
+require_once plugin_dir_path( __FILE__ ) . '/../following.php';
+require_once plugin_dir_path( __FILE__ ) . '/../followers.php';
+
+function handle_outbox( $actor_slug, $activity ) {
+    $object = validate_undo( $activity );
+    if ( is_wp_error( $object ) ) {
+        return $object;
+    }
+    $actor_id = \actors\get_actor_id( $actor_slug );
+    if ( !$actor_id ) {
+        return new \WP_Error(
+            'not_found',
+            __( 'Actor not found', 'pterotype' ),
+            array( 'status' => 404 )
+        );
+    }
+    switch ( $object['type'] ) {
+    case 'Like':
+        if ( !array_key_exists( 'object', $object ) ) {
+            return new \WP_Error(
+                'invalid_activity',
+                __( 'Expected an "object" field', 'pterotype' ),
+                array( 'status' => 400 )
+            );
+        }
+        $liked_object_url = \util\get_id( $object['object'] );
+        if ( !$liked_object_url ) {
+            break;
+        }
+        $liked_object_id = \objects\get_object_id( $liked_object_url );
+        if ( !$liked_object_id ) {
+            break;
+        }
+        \likes\delete_local_actor_like( $actor_id, $liked_object_id );
+        $like_id = \activities\get_activity_id( $object['id'] );
+        if ( !$like_id ) {
+            break;
+        }
+        \likes\delete_object_like( $liked_object_id, $like_id );
+        break;
+    case 'Block':
+        if ( !array_key_exists( 'object', $object ) ) {
+            break;
+        }
+        $blocked_object_url = \util\get_id( $object['object'] );
+        if ( !$blocked_object_url ) {
+            break;
+        }
+        $res = \blocks\delete_block( $actor_id, $blocked_object_url );
+        if ( is_wp_error( $res ) ) {
+            return $res;
+        }
+        break;
+    case 'Follow':
+        if ( !array_key_exists( 'object', $object ) ) {
+            break;
+        }
+        $follow_object_url = \util\get_id( $object['object'] );
+        if ( !$follow_object_url ) {
+            break;
+        }
+        $follow_object_id = \objects\get_object_id( $follow_object_url );
+        if ( !$follow_object_id ) {
+            break;
+        }
+        \following\reject_follow( $actor_id, $follow_object_id );
+        break;
+    // TODO I should support Undoing these as well
+    case 'Add':
+    case 'Remove':
+    case 'Accept':
+        break;
+    default:
+        break;
+    }
+    return $activity;
+}
+
+function handle_inbox( $actor_slug, $activity ) {
+    $object = validate_undo( $activity );
+    if ( is_wp_error( $object ) ) {
+        return $object;
+    }
+    $actor_id = \actors\get_actor_id( $actor_slug );
+    if ( !$actor_id ) {
+        return new \WP_Error(
+            'not_found',
+            __( 'Actor not found', 'pterotype' ),
+            array( 'status' => 404 )
+        );
+    }
+    switch( $object['type'] ) {
+    case 'Like':
+        if ( !array_key_exists( 'object', $object ) ) {
+            break;
+        }
+        if ( \objects\is_local_object( $object['object'] ) ) {
+            $object_url = \objects\get_object_id( $object['object'] );
+            if ( !$object_url ) {
+                break;
+            }
+            $object_id = \objects\get_object_id( $object_url );
+            $like_id = \activities\get_activity_id( $object['id'] );
+            if ( !$like_id ) {
+                break;
+            }
+            \likes\delete_object_like( $object_id, $like_id );
+        }
+        break;
+    case 'Follow':
+        if ( !array_key_exists( 'actor', $object ) ) {
+            break;
+        }
+        $follower = $object['actor'];
+        \followers\remove_follower( $actor_slug, $follower );
+        break;
+    case 'Accept':
+        if ( !array_key_exists( 'object', $object ) ) {
+            break;
+        }
+        $accept_object = \util\dereference_object( $object['object'] );
+        if ( is_wp_error( $object ) ) {
+            break;
+        }
+        if ( array_key_exists( 'type', $accept_object ) && $accept_object['type'] === 'Follow' ) {
+            if ( !array_key_exists( 'object', $accept_object ) ) {
+                break;
+            }
+            $followed_object_url = \util\get_id( $accept_object['object'] );
+            $followed_object_id = \objects\get_object_id( $followed_object_url );
+            if ( !$followed_object_id ) {
+                break;
+            }
+            // Put the follow request back into the PENDING state
+            \following\request_follow( $actor_id, $followed_object_id );
+        }
+        break;
+    default:
+        break;
+    }
+    return $activity;
+}
+
+function validate_undo( $activity ) {
+    if ( !array_key_exists( 'actor', $activity ) ) {
+        return new \WP_Error(
+            'invalid_activity',
+            __( 'Expected an "actor" field', 'pterotype' ),
+            array( 'status' => 400 )
+        );
+    }
+    if ( !array_key_exists( 'object', $activity ) ) {
+        return new \WP_Error(
+            'invalid_activity',
+            __( 'Expected an "object" field', 'pterotype' ),
+            array( 'status' => 400 )
+        );
+    }
+    $object = \util\dereference_object( $activity['object'] );
+    if ( is_wp_error( $object ) ) {
+        return $object;
+    }
+    if ( !array_key_exists( 'actor', $object ) ) {
+        return new \WP_Error(
+            'invalid_activity',
+            __( 'Expected a "actor" field', 'pterotype' ),
+            array( 'status' => 400 )
+        );
+    }
+    if ( !array_key_exists( 'id', $object ) ) {
+        return new \WP_Error(
+            'invalid_activity',
+            __( 'Expected an "id" field', 'pterotype' ),
+            array( 'status' => 400 )
+        );
+    }
+    if ( !\util\is_same_object( $activity['actor'], $object['actor'] ) ) {
+        return new \WP_Error(
+            'unauthorized',
+            __( 'Unauthorzed Undo activity', 'pterotype' ),
+            array( 'status' => 403 )
+        );
+    }
+    if ( !array_key_exists( 'type', $object ) ) {
+        return new \WP_Error(
+            'invalid_activity',
+            __( 'Expected a "type" field', 'pterotype' ),
+            array( 'status' => 400 )
+        );
+    }
+    return $object;
+}
+?>
diff --git a/inc/blocks.php b/inc/blocks.php
index 881b05b..a0cadac 100644
--- a/inc/blocks.php
+++ b/inc/blocks.php
@@ -17,4 +17,15 @@ function create_block( $actor_id, $blocked_actor_url ) {
         return new \WP_Error( 'db_error', __( 'Error inserting block row', 'pterotype' ) );
     }
 }
+
+function delete_block( $actor_id, $blocked_actor_url ) {
+    global $wpdb;
+    $res = $wpdb->delete(
+        'pterotype_blocks',
+        array( 'actor_id' => $actor_id, 'blocked_actor_url' => $blocked_actor_url )
+    );
+    if ( !$res ) {
+        return new \WP_Error( 'db_error', __( 'Error inserting block row', 'pterotype' ) );
+    }
+}
 ?>
diff --git a/inc/followers.php b/inc/followers.php
index f7424a5..e0857ab 100644
--- a/inc/followers.php
+++ b/inc/followers.php
@@ -35,6 +35,40 @@ function add_follower( $actor_slug, $follower ) {
     );
 }
 
+function remove_follower( $actor_slug, $follower ) {
+    global $wpdb;
+    $actor_id = \actors\get_actor_id( $actor_slug ); 
+    if ( !$actor_id ) {
+        return new \WP_Error(
+            'not_found',
+            __( 'Actor not found', 'pterotype' ),
+            array( 'status' => 404 )
+        );
+    }
+    if ( !array_key_exists( 'id', $follower ) ) {
+        return new \WP_Error(
+            'invalid_object',
+            __( 'Object must have an "id" field', 'pterotype' ),
+            array( 'status' => 400 )
+        );
+    }
+    $object_id = \objects\get_object_id( $follower['id'] );
+    if ( !$object_id ) {
+        return new \WP_Error(
+            'not_found',
+            __( 'Object not found', 'pterotype' ),
+            array( 'status' => 404 )
+        );
+    }
+    $wpdb->delete(
+        'pterotype_followers',
+        array(
+            'actor_id' => $actor_id,
+            'object_id' = $object_id,
+        );
+    );
+}
+
 function get_followers_collection( $actor_slug ) {
     global $wpdb;
     $actor_id = \actors\get_actor_id( $actor_slug ); 
diff --git a/inc/following.php b/inc/following.php
index 3a1d9ef..8f6c835 100644
--- a/inc/following.php
+++ b/inc/following.php
@@ -8,11 +8,12 @@ define( 'PTEROTYPE_FOLLOW_FOLLOWING', 'FOLLOWING' );
 
 function request_follow( $actor_id, $object_id ) {
     global $wpdb;
-    return $wpdb->insert(
+    return $wpdb->replace(
         'pterotype_following',
-        array( 'actor_id' => $actor_id,
-               'object_id' => $object_id,
-               'state' => PTEROTYPE_FOLLOW_PENDING
+        array(
+            'actor_id' => $actor_id,
+            'object_id' => $object_id,
+            'state' => PTEROTYPE_FOLLOW_PENDING
         )
     );
 }
diff --git a/inc/inbox.php b/inc/inbox.php
index 71572ec..adb16f3 100644
--- a/inc/inbox.php
+++ b/inc/inbox.php
@@ -19,6 +19,7 @@ require_once plugin_dir_path( __FILE__ ) . '/activities/follow.php';
 require_once plugin_dir_path( __FILE__ ) . '/activities/accept.php';
 require_once plugin_dir_path( __FILE__ ) . '/activities/reject.php';
 require_once plugin_dir_path( __FILE__ ) . '/activities/announce.php';
+require_once plugin_dir_path( __FILE__ ) . '/activities/undo.php';
 
 function handle_activity( $actor_slug, $activity ) {
     if ( !array_key_exists( 'type', $activity ) ) {
@@ -52,17 +53,11 @@ function handle_activity( $actor_slug, $activity ) {
     case 'Reject':
         $activity = \activities\reject\handle_inbox( $actor_slug, $activity );
         break;
-    case 'Add':
-        // TODO not yet implemented
-        break;
-    case 'Remove':
-        // TODO not yet implemented
-        break;
     case 'Announce':
         $activity = \activities\announce\handle_inbox( $actor_slug, $activity );
         break;
     case 'Undo':
-        // TODO
+        $activity = \activities\undo\handle_inbox( $actor_slug, $activity );
         break;
     }
     if ( is_wp_error( $activity ) ) {
diff --git a/inc/likes.php b/inc/likes.php
index e56adee..3b487a9 100644
--- a/inc/likes.php
+++ b/inc/likes.php
@@ -12,6 +12,15 @@ function create_local_actor_like( $actor_id, $object_id ) {
     );
 }
 
+function delete_local_actor_like( $actor_id, $object_id ) {
+    global $wpdb;
+    return $wpdb->delete(
+        'pterotype_actor_likes',
+        array( 'actor_id' => $actor_id, 'object_id' => $object_id ),
+        '%d'
+    );
+}
+
 function record_like ( $object_id, $like_id ) {
     global $wpdb;
     return $wpdb->insert(
@@ -24,6 +33,18 @@ function record_like ( $object_id, $like_id ) {
     );
 }
 
+function delete_object_like( $object_id, $like_id ) {
+    global $wpdb;
+    return $wpdb->delete(
+        'pterotype_object_likes',
+        array(
+            'object_id' => $object_id,
+            'like_id' => $like_id
+        ),
+        '%d'
+    );
+}
+
 function get_likes_collection( $object_id ) {
     global $wpdb;
     $likes = $wpdb->get_results(
diff --git a/inc/objects.php b/inc/objects.php
index da083ee..ff1ede3 100644
--- a/inc/objects.php
+++ b/inc/objects.php
@@ -240,8 +240,18 @@ function make_tombstone( $object ) {
 }
 
 function is_local_object( $object ) {
-    if ( array_key_exists( 'id', $object ) ) {
-        $parsed = parse_url( $object['id'] );
+    if ( is_array( $object ) ) {
+        if ( array_key_exists( 'id', $object ) ) {
+            $parsed = parse_url( $object['id'] );
+            if ( $parsed ) {
+                $site_host = parse_url( get_site_url() )['host'];
+                return $parsed['host'] === $site_host;
+            }
+        } else {
+            return false;
+        }
+    } else {
+        $parsed = parse_url( $object );
         if ( $parsed ) {
             $site_host = parse_url( get_site_url() )['host'];
             return $parsed['host'] === $site_host;
diff --git a/inc/outbox.php b/inc/outbox.php
index b74273e..be8e39c 100644
--- a/inc/outbox.php
+++ b/inc/outbox.php
@@ -21,6 +21,7 @@ require_once plugin_dir_path( __FILE__ ) . '/activities/delete.php';
 require_once plugin_dir_path( __FILE__ ) . '/activities/like.php';
 require_once plugin_dir_path( __FILE__ ) . '/activities/follow.php';
 require_once plugin_dir_path( __FILE__ ) . '/activities/block.php';
+require_once plugin_dir_path( __FILE__ ) . '/activities/undo.php';
 
 function handle_activity( $actor_slug, $activity ) {
     // TODO handle authentication/authorization
@@ -69,7 +70,7 @@ function handle_activity( $actor_slug, $activity ) {
         $activity = \activities\block\handle_outbox( $actor_slug, $activity );
         break;
     case 'Undo':
-        // TODO
+        $activity = \activities\undo\handle_outbox( $actor_slug, $activity );
         break;
     case 'Accept':
         $activity = \activities\accept\handle_inbox( $actor_slug, $activity );
diff --git a/inc/util.php b/inc/util.php
new file mode 100644
index 0000000..4342c68
--- /dev/null
+++ b/inc/util.php
@@ -0,0 +1,47 @@
+<?php
+namespace util;
+
+// TODO audit places throughout the repo where I need to dereference objects
+// (this is anywhere I access a field on an object, basically)
+
+function dereference_object( $object ) {
+    if ( is_array( $object ) ) {
+        return $object;
+    } else if ( filter_var( $object, FILTER_VALIDATE_URL ) ) {
+        $response = wp_remote_get( $object );
+        if ( is_wp_error( $response ) ) {
+            return $response;
+        }
+        $body = wp_remote_retrieve_body( $response );
+        if ( empty( $body ) ) {
+            return new \WP_Error(
+                'not_found',
+                __( 'The object did not dereference to a valid object', 'pterotype' ),
+                array( 'status' => 404 )
+            );
+        }
+        $body_array = json_decode( $body, true );
+        return $body_array;
+    } else {
+        return new \WP_Error(
+            'invalid_object',
+            __( 'Not a valid ActivityPub object or reference', 'pterotype' ),
+            array( 'status' => 400 )
+        );
+    }
+}
+
+function is_same_object( $object1, $object2 ) {
+    return get_id( $object1 ) === get_id( $object2 );
+}
+
+function get_id( $object ) {
+    if ( is_array( $object ) ) {
+        return array_key_exists( 'id', $object ) ?
+            $object['id'] :
+            null;
+    } else {
+        return $object;
+    }
+}
+?>