diff --git a/src/engine/blocks.js b/src/engine/blocks.js
index 3d9b44d61..86525627d 100644
--- a/src/engine/blocks.js
+++ b/src/engine/blocks.js
@@ -270,7 +270,7 @@ class Blocks {
     /**
      * Block management: change block field values
      * @param {!object} args Blockly change event to be processed
-     * @param {?Runtime} optRuntime Optional runtime to allow changeBlock to emit actions.
+     * @param {?Runtime} optRuntime Optional runtime to allow changeBlock to change VM state.
      */
     changeBlock (args, optRuntime) {
         // Validate
@@ -291,21 +291,23 @@ class Blocks {
         case 'checkbox':
             block.isMonitored = args.value;
             if (optRuntime && wasMonitored && !block.isMonitored) {
-                optRuntime.removeMonitors([{id: block.id}]);
+                optRuntime.removeMonitor(block.id);
             } else if (optRuntime && !wasMonitored && block.isMonitored) {
-                optRuntime.addMonitors(
+                optRuntime.addMonitor(
                     // Ensure that value is not undefined, since React requires it
-                    [{
+                    {
                         // @todo(dd) this will collide if multiple sprites use same block
                         id: block.id,
                         category: 'data',
                         // @todo(dd) how to handle translation here?
                         label: block.opcode,
+                        // @todo(dd) for numerical values with decimals, some countries use comma
                         value: '',
                         x: 0,
-                        // @todo(dd) place below the last monitor instead
+                        // @todo(dd) Don't require sending x and y when instantiating a
+                        // monitor. If it's not preset the GUI should decide.
                         y: 0
-                    }]
+                    }
                 );
             }
             break;
diff --git a/src/engine/execute.js b/src/engine/execute.js
index 0d263fba7..a6273fc87 100644
--- a/src/engine/execute.js
+++ b/src/engine/execute.js
@@ -91,12 +91,11 @@ const execute = function (sequencer, thread) {
                 if (thread.showVisualReport) {
                     runtime.visualReport(currentBlockId, resolvedValue);
                 }
-
                 if (thread.updateMonitor) {
-                    runtime.updateMonitors([{
+                    runtime.updateMonitor({
                         id: currentBlockId,
                         value: String(resolvedValue)
-                    }]);
+                    });
                 }
             }
             // Finished any yields.
diff --git a/src/engine/runtime.js b/src/engine/runtime.js
index 48d2da5cd..a1ab579e7 100644
--- a/src/engine/runtime.js
+++ b/src/engine/runtime.js
@@ -104,6 +104,11 @@ class Runtime extends EventEmitter {
          */
         this._cloneCounter = 0;
 
+        /**
+         * List of all monitors.
+         */
+        this._monitorState = {};
+
         /**
          * Whether the project is in "turbo mode."
          * @type {Boolean}
@@ -695,6 +700,7 @@ class Runtime extends EventEmitter {
             // @todo: Only render when this.redrawRequested or clones rendered.
             this.renderer.draw();
         }
+        this.emit(Runtime.MONITORS_UPDATE, Object.values(this._monitorState));
     }
 
     /**
@@ -874,27 +880,32 @@ class Runtime extends EventEmitter {
     }
 
     /**
-     * Emit a monitor update which adds or updates if exists the given monitors.
-     * @param {!Array} monitors Array of monitors to update.
+     * Add a monitor to the state. If the monitor already exists in the state,
+     * overwrites it.
+     * @param {!object} monitor Monitor to add.
      */
-    updateMonitors (monitors) {
-        this.emit(Runtime.MONITORS_UPDATE, monitors);
+    addMonitor (monitor) {
+        this._monitorState[monitor.id] = monitor;
     }
 
     /**
-     * Emit a monitor update which removes if exists the given monitors.
-     * @param {!Array} monitors Array of monitors to remove.
+     * Update a monitor in the state. Does nothing if the monitor does not already
+     * exist in the state.
+     * @param {!object} monitor Monitor to update.
      */
-    removeMonitors (monitors) {
-        this.emit(Runtime.MONITORS_REMOVED, monitors);
+    updateMonitor (monitor) {
+        if (this._monitorState.hasOwnProperty(monitor.id)) {
+            this._monitorState[monitor.id] = Object.assign({}, this._monitorState[monitor.id], monitor);
+        }
     }
 
     /**
-     * Emit a monitor update which adds if it doesn't already exist the given monitors.
-     * @param {!Array} monitors Array of monitors to add.
+     * Removes a monitor from the state. Does nothing if the monitor already does
+     * not exist in the state.
+     * @param {!object} monitorId ID of the monitor to remove.
      */
-    addMonitors (monitors) {
-        this.emit(Runtime.MONITORS_ADDED, monitors);
+    removeMonitor (monitorId) {
+        delete this._monitorState[monitorId];
     }
 
     /**