diff --git a/src/debugger/gui/KeyboardWidget.cxx b/src/debugger/gui/KeyboardWidget.cxx
index 61ee42692..e0ca3ba05 100644
--- a/src/debugger/gui/KeyboardWidget.cxx
+++ b/src/debugger/gui/KeyboardWidget.cxx
@@ -54,7 +54,7 @@ KeyboardWidget::KeyboardWidget(GuiObject* boss, const GUI::Font& font,
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 void KeyboardWidget::loadConfig()
 {
-  Event& event = instance().eventHandler().event();
+  const Event& event = instance().eventHandler().event();
   for(int i = 0; i < 12; ++i)
     myBox[i]->setState(event.get(myEvent[i]));
 }
diff --git a/src/emucore/AmigaMouse.hxx b/src/emucore/AmigaMouse.hxx
index 0e042b294..09615a216 100644
--- a/src/emucore/AmigaMouse.hxx
+++ b/src/emucore/AmigaMouse.hxx
@@ -30,7 +30,7 @@ class AmigaMouse : public PointingDevice
       @param event  The event object to use for events
       @param system The system using this controller
     */
-    AmigaMouse(Jack jack, Event& event, const System& system)
+    AmigaMouse(Jack jack, const Event& event, const System& system)
       : PointingDevice(jack, event, system, Controller::AmigaMouse,
         trackballSensitivity) { }
     virtual ~AmigaMouse() = default;
diff --git a/src/emucore/AtariMouse.hxx b/src/emucore/AtariMouse.hxx
index 9081855fd..0ad879afc 100644
--- a/src/emucore/AtariMouse.hxx
+++ b/src/emucore/AtariMouse.hxx
@@ -30,7 +30,7 @@ class AtariMouse : public PointingDevice
       @param event  The event object to use for events
       @param system The system using this controller
     */
-    AtariMouse(Jack jack, Event& event, const System& system)
+    AtariMouse(Jack jack, const Event& event, const System& system)
       : PointingDevice(jack, event, system, Controller::AtariMouse,
         trackballSensitivity) { }
     virtual ~AtariMouse() = default;
diff --git a/src/emucore/AtariVox.cxx b/src/emucore/AtariVox.cxx
index 9e0773c46..5cdf285cb 100644
--- a/src/emucore/AtariVox.cxx
+++ b/src/emucore/AtariVox.cxx
@@ -20,7 +20,7 @@
 #include "AtariVox.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-AtariVox::AtariVox(Jack jack, Event& event, const System& system,
+AtariVox::AtariVox(Jack jack, const Event& event, const System& system,
                    const SerialPort& port, const string& portname,
                    const string& eepromfile)
   : SaveKey(jack, event, system, eepromfile, Controller::AtariVox),
diff --git a/src/emucore/AtariVox.hxx b/src/emucore/AtariVox.hxx
index 8707fb848..cec2c72aa 100644
--- a/src/emucore/AtariVox.hxx
+++ b/src/emucore/AtariVox.hxx
@@ -45,7 +45,7 @@ class AtariVox : public SaveKey
       @param portname   Name of the port used for reading and writing
       @param eepromfile The file containing the EEPROM data
     */
-    AtariVox(Jack jack, Event& event, const System& system,
+    AtariVox(Jack jack, const Event& event, const System& system,
              const SerialPort& port, const string& portname,
              const string& eepromfile);
     virtual ~AtariVox() = default;
diff --git a/src/emucore/Booster.cxx b/src/emucore/Booster.cxx
index 0a9922c83..e09859ae1 100644
--- a/src/emucore/Booster.cxx
+++ b/src/emucore/Booster.cxx
@@ -19,7 +19,7 @@
 #include "Booster.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-BoosterGrip::BoosterGrip(Jack jack, Event& event, const System& system)
+BoosterGrip::BoosterGrip(Jack jack, const Event& event, const System& system)
   : Controller(jack, event, system, Controller::BoosterGrip),
     myControlID(-1)
 {
diff --git a/src/emucore/Booster.hxx b/src/emucore/Booster.hxx
index c5799bc5a..a89c94fff 100644
--- a/src/emucore/Booster.hxx
+++ b/src/emucore/Booster.hxx
@@ -38,7 +38,7 @@ class BoosterGrip : public Controller
       @param event  The event object to use for events
       @param system The system using this controller
     */
-    BoosterGrip(Jack jack, Event& event, const System& system);
+    BoosterGrip(Jack jack, const Event& event, const System& system);
     virtual ~BoosterGrip() = default;
 
   public:
diff --git a/src/emucore/CompuMate.cxx b/src/emucore/CompuMate.cxx
index a53315f5a..42c385105 100644
--- a/src/emucore/CompuMate.cxx
+++ b/src/emucore/CompuMate.cxx
@@ -20,7 +20,7 @@
 #include "CompuMate.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-CompuMate::CompuMate(const Console& console, Event& event,
+CompuMate::CompuMate(const Console& console, const Event& event,
                      const System& system)
   : myConsole(console),
     myColumn(0),
diff --git a/src/emucore/CompuMate.hxx b/src/emucore/CompuMate.hxx
index 87f1fef13..01d7c95ba 100644
--- a/src/emucore/CompuMate.hxx
+++ b/src/emucore/CompuMate.hxx
@@ -50,7 +50,7 @@ class CompuMate
       @param event    The event object to use for events
       @param system   The system using this controller
     */
-    CompuMate(const Console& console, Event& event, const System& system);
+    CompuMate(const Console& console, const Event& event, const System& system);
     virtual ~CompuMate() = default;  // Controllers are deleted outside this class
 
     /**
@@ -98,7 +98,7 @@ class CompuMate
           @param event    The event object to use for events
           @param system   The system using this controller
         */
-        CMControl(class CompuMate& handler, Controller::Jack jack, Event& event,
+        CMControl(class CompuMate& handler, Controller::Jack jack, const Event& event,
                   const System& system)
           : Controller(jack, event, system, Controller::CompuMate),
             myHandler(handler) { }
diff --git a/src/emucore/Console.hxx b/src/emucore/Console.hxx
index 4c9fa25ee..a8ab18785 100644
--- a/src/emucore/Console.hxx
+++ b/src/emucore/Console.hxx
@@ -372,7 +372,7 @@ class Console : public Serializable
     OSystem& myOSystem;
 
     // Reference to the event object to use
-    Event& myEvent;
+    const Event& myEvent;
 
     // Properties for the game
     Properties myProperties;
diff --git a/src/emucore/Control.cxx b/src/emucore/Control.cxx
index a7a91679a..c522b3947 100644
--- a/src/emucore/Control.cxx
+++ b/src/emucore/Control.cxx
@@ -21,7 +21,7 @@
 #include "Control.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-Controller::Controller(Jack jack, Event& event, const System& system,
+Controller::Controller(Jack jack, const Event& event, const System& system,
                        Type type)
   : myJack(jack),
     myEvent(event),
diff --git a/src/emucore/Control.hxx b/src/emucore/Control.hxx
index 2e0461d32..45a6bdd7c 100644
--- a/src/emucore/Control.hxx
+++ b/src/emucore/Control.hxx
@@ -108,7 +108,7 @@ class Controller : public Serializable
       @param system The system using this controller
       @param type   The type for this controller
     */
-    Controller(Jack jack, Event& event, const System& system,
+    Controller(Jack jack, const Event& event, const System& system,
                Type type);
     virtual ~Controller() = default;
 
@@ -275,7 +275,7 @@ class Controller : public Serializable
     const Jack myJack;
 
     /// Reference to the event object this controller uses
-    Event& myEvent;
+    const Event& myEvent;
 
     /// Pointer to the System object (used for timing purposes)
     const System& mySystem;
diff --git a/src/emucore/Driving.cxx b/src/emucore/Driving.cxx
index be24c2f85..e209b723f 100644
--- a/src/emucore/Driving.cxx
+++ b/src/emucore/Driving.cxx
@@ -21,7 +21,7 @@
 #include "Driving.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-Driving::Driving(Jack jack, Event& event, const System& system)
+Driving::Driving(Jack jack, const Event& event, const System& system)
   : Controller(jack, event, system, Controller::Driving),
     myCounter(0),
     myGrayIndex(0),
diff --git a/src/emucore/Driving.hxx b/src/emucore/Driving.hxx
index 41b7343c1..0925cef25 100644
--- a/src/emucore/Driving.hxx
+++ b/src/emucore/Driving.hxx
@@ -37,7 +37,7 @@ class Driving : public Controller
       @param event  The event object to use for events
       @param system The system using this controller
     */
-    Driving(Jack jack, Event& event, const System& system);
+    Driving(Jack jack, const Event& event, const System& system);
     virtual ~Driving() = default;
 
   public:
diff --git a/src/emucore/Event.hxx b/src/emucore/Event.hxx
index 582881a35..75c40a3ea 100644
--- a/src/emucore/Event.hxx
+++ b/src/emucore/Event.hxx
@@ -24,7 +24,7 @@
 #include "StellaKeys.hxx"
 
 /**
-  @author  Bradford W. Mott, Stephen Anthony
+  @author  Stephen Anthony, Christian Speckner
 */
 class Event
 {
@@ -89,9 +89,10 @@ class Event
           : myKeyTable(keyTable),
             myMutex(mutex),
             myIsEnabled(true)
-        {}
+        {
+        }
 
-        bool operator[](int type) {
+        bool operator[](int type) const {
           if (!myIsEnabled) return false;
 
           std::lock_guard<std::mutex> lock(myMutex);
@@ -122,7 +123,7 @@ class Event
     /**
       Get the value associated with the event of the specified type.
     */
-    Int32 get(Type type) {
+    Int32 get(Type type) const {
       std::lock_guard<std::mutex> lock(myMutex);
 
       return myValues[type];
@@ -154,7 +155,7 @@ class Event
     /**
       Get the keytable associated with this event.
     */
-    KeyTable getKeys() { return KeyTable(myKeyTable, myMutex); }
+    KeyTable getKeys() const { return KeyTable(myKeyTable, myMutex); }
 
     /**
       Set the value associated with the event of the specified type.
@@ -189,7 +190,7 @@ class Event
     // Array of keyboard key states
     bool myKeyTable[KBDK_LAST];
 
-    std::mutex myMutex;
+    mutable std::mutex myMutex;
 
   private:
     // Following constructors and assignment operators not supported
diff --git a/src/emucore/EventHandler.hxx b/src/emucore/EventHandler.hxx
index e8344bc53..a6d32ce98 100644
--- a/src/emucore/EventHandler.hxx
+++ b/src/emucore/EventHandler.hxx
@@ -63,7 +63,7 @@ class EventHandler
 
       @return The event object
     */
-    Event& event() { return myEvent; }
+    const Event& event() const { return myEvent; }
 
     /**
       Initialize state of this eventhandler.
diff --git a/src/emucore/Genesis.cxx b/src/emucore/Genesis.cxx
index 7085228ae..74de86906 100644
--- a/src/emucore/Genesis.cxx
+++ b/src/emucore/Genesis.cxx
@@ -19,7 +19,7 @@
 #include "Genesis.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-Genesis::Genesis(Jack jack, Event& event, const System& system)
+Genesis::Genesis(Jack jack, const Event& event, const System& system)
   : Controller(jack, event, system, Controller::Genesis),
     myControlID(-1)
 {
diff --git a/src/emucore/Genesis.hxx b/src/emucore/Genesis.hxx
index 323fb124c..c94783358 100644
--- a/src/emucore/Genesis.hxx
+++ b/src/emucore/Genesis.hxx
@@ -40,7 +40,7 @@ class Genesis : public Controller
       @param event  The event object to use for events
       @param system The system using this controller
     */
-    Genesis(Jack jack, Event& event, const System& system);
+    Genesis(Jack jack, const Event& event, const System& system);
     virtual ~Genesis() = default;
 
   public:
diff --git a/src/emucore/Joystick.cxx b/src/emucore/Joystick.cxx
index 134228162..ccfaa1faf 100644
--- a/src/emucore/Joystick.cxx
+++ b/src/emucore/Joystick.cxx
@@ -19,7 +19,7 @@
 #include "Joystick.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-Joystick::Joystick(Jack jack, Event& event, const System& system)
+Joystick::Joystick(Jack jack, const Event& event, const System& system)
   : Controller(jack, event, system, Controller::Joystick),
     myControlID(-1)
 {
diff --git a/src/emucore/Joystick.hxx b/src/emucore/Joystick.hxx
index f3c1b1129..228ed9109 100644
--- a/src/emucore/Joystick.hxx
+++ b/src/emucore/Joystick.hxx
@@ -37,7 +37,7 @@ class Joystick : public Controller
       @param event  The event object to use for events
       @param system The system using this controller
     */
-    Joystick(Jack jack, Event& event, const System& system);
+    Joystick(Jack jack, const Event& event, const System& system);
     virtual ~Joystick() = default;
 
   public:
diff --git a/src/emucore/Keyboard.cxx b/src/emucore/Keyboard.cxx
index 301165efe..996053d32 100644
--- a/src/emucore/Keyboard.cxx
+++ b/src/emucore/Keyboard.cxx
@@ -19,7 +19,7 @@
 #include "Keyboard.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-Keyboard::Keyboard(Jack jack, Event& event, const System& system)
+Keyboard::Keyboard(Jack jack, const Event& event, const System& system)
   : Controller(jack, event, system, Controller::Keyboard)
 {
   if(myJack == Left)
diff --git a/src/emucore/Keyboard.hxx b/src/emucore/Keyboard.hxx
index 46c10cbc8..aa5ca23f7 100644
--- a/src/emucore/Keyboard.hxx
+++ b/src/emucore/Keyboard.hxx
@@ -37,7 +37,7 @@ class Keyboard : public Controller
       @param event  The event object to use for events
       @param system The system using this controller
     */
-    Keyboard(Jack jack, Event& event, const System& system);
+    Keyboard(Jack jack, const Event& event, const System& system);
     virtual ~Keyboard() = default;
 
   public:
diff --git a/src/emucore/KidVid.cxx b/src/emucore/KidVid.cxx
index fd5b713de..2d44259c8 100644
--- a/src/emucore/KidVid.cxx
+++ b/src/emucore/KidVid.cxx
@@ -21,7 +21,7 @@
 #include "KidVid.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-KidVid::KidVid(Jack jack, Event& event, const System& system,
+KidVid::KidVid(Jack jack, const Event& event, const System& system,
                const string& rommd5)
   : Controller(jack, event, system, Controller::KidVid),
     myEnabled(myJack == Right),
diff --git a/src/emucore/KidVid.hxx b/src/emucore/KidVid.hxx
index ce26b7373..8365a3d15 100644
--- a/src/emucore/KidVid.hxx
+++ b/src/emucore/KidVid.hxx
@@ -47,7 +47,7 @@ class KidVid : public Controller
       @param system The system using this controller
       @param md5sum The md5 of the ROM using this controller
     */
-    KidVid(Jack jack, Event& event, const System& system,
+    KidVid(Jack jack, const Event& event, const System& system,
            const string& md5sum);
     virtual ~KidVid();
 
diff --git a/src/emucore/MindLink.cxx b/src/emucore/MindLink.cxx
index b715a1aa9..f2b13d9af 100644
--- a/src/emucore/MindLink.cxx
+++ b/src/emucore/MindLink.cxx
@@ -19,7 +19,7 @@
 #include "MindLink.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-MindLink::MindLink(Jack jack, Event& event, const System& system)
+MindLink::MindLink(Jack jack, const Event& event, const System& system)
   : Controller(jack, event, system, Controller::MindLink),
     myMindlinkPos(0x2800),
     myMindlinkShift(1),
diff --git a/src/emucore/MindLink.hxx b/src/emucore/MindLink.hxx
index c43fb20a3..15c7c96ba 100644
--- a/src/emucore/MindLink.hxx
+++ b/src/emucore/MindLink.hxx
@@ -47,7 +47,7 @@ class MindLink : public Controller
       @param event  The event object to use for events
       @param system The system using this controller
     */
-    MindLink(Jack jack, Event& event, const System& system);
+    MindLink(Jack jack, const Event& event, const System& system);
     virtual ~MindLink() = default;
 
   public:
diff --git a/src/emucore/Paddles.cxx b/src/emucore/Paddles.cxx
index 8511d696d..f6bca2b46 100644
--- a/src/emucore/Paddles.cxx
+++ b/src/emucore/Paddles.cxx
@@ -19,7 +19,7 @@
 #include "Paddles.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-Paddles::Paddles(Jack jack, Event& event, const System& system,
+Paddles::Paddles(Jack jack, const Event& event, const System& system,
                  bool swappaddle, bool swapaxis, bool swapdir)
   : Controller(jack, event, system, Controller::Paddles),
     myMPaddleID(-1),
diff --git a/src/emucore/Paddles.hxx b/src/emucore/Paddles.hxx
index 7ea3c6cbe..4a206a5d7 100644
--- a/src/emucore/Paddles.hxx
+++ b/src/emucore/Paddles.hxx
@@ -43,7 +43,7 @@ class Paddles : public Controller
                         causes movement (lesser axis values cause paddle
                         resistance to decrease instead of increase)
     */
-    Paddles(Jack jack, Event& event, const System& system,
+    Paddles(Jack jack, const Event& event, const System& system,
             bool swappaddle, bool swapaxis, bool swapdir);
     virtual ~Paddles() = default;
 
diff --git a/src/emucore/PointingDevice.cxx b/src/emucore/PointingDevice.cxx
index 327ae96fa..346e06b5a 100644
--- a/src/emucore/PointingDevice.cxx
+++ b/src/emucore/PointingDevice.cxx
@@ -23,7 +23,7 @@
 #include "PointingDevice.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-PointingDevice::PointingDevice(Jack jack, Event& event,
+PointingDevice::PointingDevice(Jack jack, const Event& event,
                                const System& system, Controller::Type type,
                                float sensitivity)
   : Controller(jack, event, system, type),
diff --git a/src/emucore/PointingDevice.hxx b/src/emucore/PointingDevice.hxx
index 8cedc26f4..8ff501c11 100644
--- a/src/emucore/PointingDevice.hxx
+++ b/src/emucore/PointingDevice.hxx
@@ -35,7 +35,7 @@ class PointingDevice : public Controller
   friend class PointingDeviceWidget;
 
   public:
-    PointingDevice(Jack jack, Event& event,
+    PointingDevice(Jack jack, const Event& event,
                    const System& system, Controller::Type type,
                    float sensitivity);
     virtual ~PointingDevice() = default;
diff --git a/src/emucore/SaveKey.cxx b/src/emucore/SaveKey.cxx
index 4a0359fa5..0e0acde8e 100644
--- a/src/emucore/SaveKey.cxx
+++ b/src/emucore/SaveKey.cxx
@@ -20,7 +20,7 @@
 #include "SaveKey.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-SaveKey::SaveKey(Jack jack, Event& event, const System& system,
+SaveKey::SaveKey(Jack jack, const Event& event, const System& system,
                  const string& eepromfile, Type type)
   : Controller(jack, event, system, type)
 {
@@ -30,7 +30,7 @@ SaveKey::SaveKey(Jack jack, Event& event, const System& system,
 }
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-SaveKey::SaveKey(Jack jack, Event& event, const System& system,
+SaveKey::SaveKey(Jack jack, const Event& event, const System& system,
                  const string& eepromfile)
   : SaveKey(jack, event, system, eepromfile, Controller::SaveKey)
 {
diff --git a/src/emucore/SaveKey.hxx b/src/emucore/SaveKey.hxx
index 0415e7e9f..ab98c8029 100644
--- a/src/emucore/SaveKey.hxx
+++ b/src/emucore/SaveKey.hxx
@@ -42,7 +42,7 @@ class SaveKey : public Controller
       @param system     The system using this controller
       @param eepromfile The file containing the EEPROM data
     */
-    SaveKey(Jack jack, Event& event, const System& system,
+    SaveKey(Jack jack, const Event& event, const System& system,
             const string& eepromfile);
     virtual ~SaveKey();
 
@@ -51,7 +51,7 @@ class SaveKey : public Controller
       Delegating constructor currently used by both this class and classes
       that inherit from SaveKey (currently, AtariVox)
     */
-    SaveKey(Jack jack, Event& event, const System& system,
+    SaveKey(Jack jack, const Event& event, const System& system,
             const string& eepromfile, Type type);
 
   public:
diff --git a/src/emucore/Switches.cxx b/src/emucore/Switches.cxx
index f8dabd93b..28f90c47e 100644
--- a/src/emucore/Switches.cxx
+++ b/src/emucore/Switches.cxx
@@ -21,7 +21,7 @@
 #include "Switches.hxx"
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-Switches::Switches(Event& event, const Properties& properties,
+Switches::Switches(const Event& event, const Properties& properties,
                    const Settings& settings)
   : myEvent(event),
     mySwitches(0xFF),
diff --git a/src/emucore/Switches.hxx b/src/emucore/Switches.hxx
index 4ba261061..b18a7bde2 100644
--- a/src/emucore/Switches.hxx
+++ b/src/emucore/Switches.hxx
@@ -46,7 +46,7 @@ class Switches : public Serializable
       @param props The ROM properties to use for the currently enabled ROM
       @param settings The settings used by the system
     */
-    Switches(Event& event, const Properties& props, const Settings& settings);
+    Switches(const Event& event, const Properties& props, const Settings& settings);
     virtual ~Switches() = default;
 
   public:
@@ -124,7 +124,7 @@ class Switches : public Serializable
 
   private:
     // Reference to the event object to use
-    Event& myEvent;
+    const Event& myEvent;
 
     // State of the console switches
     uInt8 mySwitches;
diff --git a/src/emucore/TrakBall.hxx b/src/emucore/TrakBall.hxx
index ea7123358..038b9f2c3 100644
--- a/src/emucore/TrakBall.hxx
+++ b/src/emucore/TrakBall.hxx
@@ -30,7 +30,7 @@ class TrakBall : public PointingDevice
       @param event  The event object to use for events
       @param system The system using this controller
     */
-    TrakBall(Jack jack, Event& event, const System& system)
+    TrakBall(Jack jack, const Event& event, const System& system)
       : PointingDevice(jack, event, system, Controller::TrakBall,
         trackballSensitivity) { }
     virtual ~TrakBall() = default;