diff --git a/src/engine/blocks.js b/src/engine/blocks.js
index 92a2d58e7..7564d3ca9 100644
--- a/src/engine/blocks.js
+++ b/src/engine/blocks.js
@@ -226,7 +226,8 @@ class Blocks {
                 id: e.blockId,
                 element: e.element,
                 name: e.name,
-                value: e.newValue
+                value: e.newValue,
+                isSpriteSpecific: e.isSpriteSpecific
             }, optRuntime);
             break;
         case 'move':
@@ -326,12 +327,14 @@ class Blocks {
             break;
         case 'checkbox':
             block.isMonitored = args.value;
+            block.targetId = args.isSpriteSpecific ? this._getTargetIdFromBlockId(block.id) : null;
             if (optRuntime && wasMonitored && !block.isMonitored) {
                 optRuntime.requestRemoveMonitor(block.id);
             } else if (optRuntime && !wasMonitored && block.isMonitored) {
                 optRuntime.requestAddMonitor(MonitorRecord({
                     // @todo(vm#564) this will collide if multiple sprites use same block
                     id: block.id,
+                    spriteName: block.targetId ? optRuntime.getTargetById(block.targetId).getName() : null,
                     opcode: block.opcode,
                     params: this._getBlockParams(block),
                     // @todo(vm#565) for numerical values with decimals, some countries use comma
@@ -406,12 +409,17 @@ class Blocks {
     runAllMonitored (runtime) {
         Object.keys(this._blocks).forEach(blockId => {
             if (this.getBlock(blockId).isMonitored) {
-                // @todo handle specific targets (e.g. apple x position)
-                runtime.addMonitorScript(blockId);
+                const targetId = this.getBlock(blockId).targetId;
+                runtime.addMonitorScript(blockId, targetId ? runtime.getTargetById(targetId) : null);
             }
         });
     }
 
+    _getTargetIdFromBlockId (blockId) {
+        // First word of block ID. See makeToolboxXML in scratch-gui
+        return blockId.split('_')[0];
+    }
+
     /**
      * Block management: delete blocks and their associated scripts.
      * @param {!object} e Blockly delete event to be processed.
diff --git a/src/engine/execute.js b/src/engine/execute.js
index df9b5964b..fe1688e6a 100644
--- a/src/engine/execute.js
+++ b/src/engine/execute.js
@@ -68,8 +68,10 @@ const handleReport = function (
                 sequencer.runtime.visualReport(currentBlockId, resolvedValue);
             }
             if (thread.updateMonitor) {
+                const targetId = sequencer.runtime.monitorBlocks.getBlock(currentBlockId).targetId;
                 sequencer.runtime.requestUpdateMonitor(Map({
                     id: currentBlockId,
+                    spriteName: targetId ? sequencer.runtime.getTargetById(targetId).getName() : null,
                     value: String(resolvedValue)
                 }));
             }
diff --git a/src/engine/monitor-record.js b/src/engine/monitor-record.js
index 76ed08489..40e67c5e7 100644
--- a/src/engine/monitor-record.js
+++ b/src/engine/monitor-record.js
@@ -2,6 +2,8 @@ const {Record} = require('immutable');
 
 const MonitorRecord = Record({
     id: null,
+    /** Present only if the monitor is sprite-specific, such as x position */
+    spriteName: null,
     opcode: null,
     value: null,
     params: null
diff --git a/src/engine/runtime.js b/src/engine/runtime.js
index 56fceadfc..06d068bcd 100644
--- a/src/engine/runtime.js
+++ b/src/engine/runtime.js
@@ -838,7 +838,7 @@ class Runtime extends EventEmitter {
     /**
      * Enqueue a script that when finished will update the monitor for the block.
      * @param {!string} topBlockId ID of block that starts the script.
-     * @param {?string} optTarget target ID for target to run script on. If not supplied, uses editing target.
+     * @param {?string} optTarget target Target to run script on. If not supplied, uses editing target.
      */
     addMonitorScript (topBlockId, optTarget) {
         if (!optTarget) optTarget = this._editingTarget;
@@ -1260,7 +1260,7 @@ class Runtime extends EventEmitter {
      * @param {!MonitorRecord} monitor Monitor to add.
      */
     requestAddMonitor (monitor) {
-        this._monitorState = this._monitorState.set(monitor.id, monitor);
+        this._monitorState = this._monitorState.set(monitor.get('id'), monitor);
     }
 
     /**
@@ -1271,9 +1271,10 @@ class Runtime extends EventEmitter {
      *     the old monitor will keep its old value.
      */
     requestUpdateMonitor (monitor) {
-        if (this._monitorState.has(monitor.get('id'))) {
+        const id = monitor.get('id');
+        if (this._monitorState.has(id)) {
             this._monitorState =
-                this._monitorState.set(monitor.get('id'), this._monitorState.get(monitor.get('id')).merge(monitor));
+                this._monitorState.set(id, this._monitorState.get(id).merge(monitor));
         }
     }
 
diff --git a/src/util/uid.js b/src/util/uid.js
index fd4c41e87..34e8eaf58 100644
--- a/src/util/uid.js
+++ b/src/util/uid.js
@@ -6,9 +6,10 @@
  * Legal characters for the unique ID.
  * Should be all on a US keyboard.  No XML special characters or control codes.
  * Removed $ due to issue 251.
+ * Removed _ which denotes word separation in XML.
  * @private
  */
-const soup_ = '!#%()*+,-./:;=?@[]^_`{|}~' +
+const soup_ = '!#%()*+,-./:;=?@[]^`{|}~' +
     'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
 
 /**