diff --git a/dll/appticket.h b/dll/appticket.h
index 94f92e7..1768278 100644
--- a/dll/appticket.h
+++ b/dll/appticket.h
@@ -1,4 +1,7 @@
 
+#ifndef APPTICKET_H
+#define APPTICKET_H
+
 struct AppTicketV1
 {
     // Total ticket size - 16
@@ -297,4 +300,6 @@ public:
 
         return buffer;
     }
-};
\ No newline at end of file
+};
+
+#endif // APPTICKET_H
diff --git a/dll/steam_HTMLsurface.h b/dll/steam_HTMLsurface.h
index 3b10b26..a35df54 100644
--- a/dll/steam_HTMLsurface.h
+++ b/dll/steam_HTMLsurface.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_HTMLSURFACE_H
+#define INCLUDED_STEAM_HTMLSURFACE_H
+
 class Steam_HTMLsurface :
 public ISteamHTMLSurface001,
 public ISteamHTMLSurface002,
@@ -337,3 +340,5 @@ void FileLoadDialogResponse( HHTMLBrowser unBrowserHandle, const char **pchSelec
 }
 
 };
+
+#endif // INCLUDED_STEAM_HTMLSURFACE_H
diff --git a/dll/steam_applist.h b/dll/steam_applist.h
index 10f872e..1feb871 100644
--- a/dll/steam_applist.h
+++ b/dll/steam_applist.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_APPLIST_H
+#define INCLUDED_STEAM_APPLIST_H
+
 class Steam_Applist : public ISteamAppList
 {
 public:
@@ -28,3 +31,5 @@ public:
 
 	int GetAppBuildId( AppId_t nAppID ); // return the buildid of this app, may change at any time based on backend updates to the game
 };
+
+#endif // INCLUDED_STEAM_APPLIST_H
diff --git a/dll/steam_apps.h b/dll/steam_apps.h
index 1a4f206..eab3756 100644
--- a/dll/steam_apps.h
+++ b/dll/steam_apps.h
@@ -1,5 +1,8 @@
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_APPS_H
+#define INCLUDED_STEAM_APPS_H
+
 class Steam_Apps :
 public ISteamApps001,
 public ISteamApps002,
@@ -110,3 +113,5 @@ public:
 	// set current DLC AppID being played (or 0 if none). Allows Steam to track usage of major DLC extensions
     bool SetDlcContext( AppId_t nAppID );
 };
+
+#endif // INCLUDED_STEAM_APPS_H
diff --git a/dll/steam_client.h b/dll/steam_client.h
index 37c4a85..9de5f95 100644
--- a/dll/steam_client.h
+++ b/dll/steam_client.h
@@ -53,6 +53,9 @@
 
 #include "../overlay_experimental/steam_overlay.h"
 
+#ifndef INCLUDED_STEAM_CLIENT_H
+#define INCLUDED_STEAM_CLIENT_H
+
 enum Steam_Pipe {
     NO_USER,
     CLIENT,
@@ -303,3 +306,5 @@ public:
 
     void DestroyAllInterfaces();
 };
+
+#endif // INCLUDED_STEAM_CLIENT_H
diff --git a/dll/steam_controller.h b/dll/steam_controller.h
index db04d60..b192b1c 100644
--- a/dll/steam_controller.h
+++ b/dll/steam_controller.h
@@ -16,6 +16,10 @@
    <http://www.gnu.org/licenses/>.  */
 
 #include "base.h"
+
+#ifndef INCLUDED_STEAM_CONTROLLER_H
+#define INCLUDED_STEAM_CONTROLLER_H
+
 #ifndef CONTROLLER_SUPPORT
 inline void GamepadInit(void) {}
 inline void GamepadShutdown(void) {}
@@ -1224,3 +1228,5 @@ void RunCallbacks()
 }
 
 };
+
+#endif // INCLUDED_STEAM_CONTROLLER_H
diff --git a/dll/steam_game_coordinator.h b/dll/steam_game_coordinator.h
index 1ba2b88..79f0edb 100644
--- a/dll/steam_game_coordinator.h
+++ b/dll/steam_game_coordinator.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_GAME_COORDINATOR_H
+#define INCLUDED_STEAM_GAME_COORDINATOR_H
+
 class Steam_Game_Coordinator :
 public ISteamGameCoordinator
 {
@@ -152,3 +155,5 @@ void Callback(Common_Message *msg)
 }
 
 };
+
+#endif // INCLUDED_STEAM_GAME_COORDINATOR_H
diff --git a/dll/steam_gamesearch.h b/dll/steam_gamesearch.h
index 2254f01..dd06b6c 100644
--- a/dll/steam_gamesearch.h
+++ b/dll/steam_gamesearch.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_GAMESEARCH_H
+#define INCLUDED_STEAM_GAMESEARCH_H
+
 class Steam_Game_Search :
 public ISteamGameSearch
 {
@@ -213,3 +216,5 @@ void Callback(Common_Message *msg)
 }
 
 };
+
+#endif // INCLUDED_STEAM_GAMESEARCH_H
diff --git a/dll/steam_gameserver.h b/dll/steam_gameserver.h
index 6cd9b70..8f385f2 100644
--- a/dll/steam_gameserver.h
+++ b/dll/steam_gameserver.h
@@ -16,7 +16,10 @@
    <http://www.gnu.org/licenses/>.  */
 
 #include "base.h"
- 
+
+#ifndef INCLUDED_STEAM_GAMESERVER_H
+#define INCLUDED_STEAM_GAMESERVER_H
+
 //-----------------------------------------------------------------------------
 // Purpose: Functions for authenticating users via Steam to play on a game server
 //-----------------------------------------------------------------------------
@@ -353,3 +356,5 @@ public:
     //
     void RunCallbacks();
 };
+
+#endif // INCLUDED_STEAM_GAMESERVER_H
diff --git a/dll/steam_gameserverstats.h b/dll/steam_gameserverstats.h
index 8efde54..b11e94a 100644
--- a/dll/steam_gameserverstats.h
+++ b/dll/steam_gameserverstats.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
  
+#ifndef INCLUDED_STEAM_GAMESERVERSTATS_H
+#define INCLUDED_STEAM_GAMESERVERSTATS_H
+ 
 //-----------------------------------------------------------------------------
 // Purpose: Functions for authenticating users via Steam to play on a game server
 //-----------------------------------------------------------------------------
@@ -61,3 +64,5 @@ public:
 	STEAM_CALL_RESULT( GSStatsStored_t )
 	SteamAPICall_t StoreUserStats( CSteamID steamIDUser );
 };
+
+#endif // INCLUDED_STEAM_GAMESERVERSTATS_H
diff --git a/dll/steam_http.h b/dll/steam_http.h
index 7e33f2c..c30d399 100644
--- a/dll/steam_http.h
+++ b/dll/steam_http.h
@@ -17,6 +17,8 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_HTTP_H
+#define INCLUDED_STEAM_HTTP_H
 
 struct Steam_Http_Request {
 	HTTPRequestHandle handle;
@@ -152,3 +154,5 @@ public:
 	// Check if the reason the request failed was because we timed it out (rather than some harder failure)
 	bool GetHTTPRequestWasTimedOut( HTTPRequestHandle hRequest, bool *pbWasTimedOut );
 };
+
+#endif // INCLUDED_STEAM_HTTP_H
diff --git a/dll/steam_inventory.h b/dll/steam_inventory.h
index 4d30f20..699649c 100644
--- a/dll/steam_inventory.h
+++ b/dll/steam_inventory.h
@@ -17,6 +17,9 @@
 
 #include "base.h" // For SteamItemDef_t
 
+#ifndef INCLUDED_STEAM_INVENTORY_H
+#define INCLUDED_STEAM_INVENTORY_H
+
 struct Steam_Inventory_Requests {
     double timeout = 0.1;
     bool done = false;
@@ -964,3 +967,5 @@ void RunCallbacks()
 }
 
 };
+
+#endif // INCLUDED_STEAM_INVENTORY_H
diff --git a/dll/steam_masterserver_updater.h b/dll/steam_masterserver_updater.h
index d17ccd8..c08a6b6 100644
--- a/dll/steam_masterserver_updater.h
+++ b/dll/steam_masterserver_updater.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_MASTERSERVER_UPDATER_H
+#define INCLUDED_STEAM_MASTERSERVER_UPDATER_H
+
 class Steam_Masterserver_Updater :
 public ISteamMasterServerUpdater
 {
@@ -300,3 +303,5 @@ void Callback(Common_Message *msg)
 }
 
 };
+
+#endif // INCLUDED_STEAM_MASTERSERVER_UPDATER_H
diff --git a/dll/steam_matchmaking.h b/dll/steam_matchmaking.h
index 94bd026..d3e1103 100644
--- a/dll/steam_matchmaking.h
+++ b/dll/steam_matchmaking.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_MATCHMAKING_H
+#define INCLUDED_STEAM_MATCHMAKING_H
+
 #define SEND_LOBBY_RATE 5.0
 
 #define PENDING_JOIN_TIMEOUT 10.0
@@ -1547,3 +1550,5 @@ void Callback(Common_Message *msg)
 
 
 };
+
+#endif // INCLUDED_STEAM_MATCHMAKING_H
diff --git a/dll/steam_matchmaking_servers.h b/dll/steam_matchmaking_servers.h
index 21a643f..0650467 100644
--- a/dll/steam_matchmaking_servers.h
+++ b/dll/steam_matchmaking_servers.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_MATCHMAKING_SERVERS_H
+#define INCLUDED_STEAM_MATCHMAKING_SERVERS_H
+
 #define SERVER_TIMEOUT 10.0
 #define DIRECT_IP_DELAY 0.05
 
@@ -223,3 +226,5 @@ public:
     void Callback(Common_Message *msg);
     void server_details(Gameserver *g, gameserveritem_t *server);
 };
+
+#endif // INCLUDED_STEAM_MATCHMAKING_SERVERS_H
diff --git a/dll/steam_music.h b/dll/steam_music.h
index 37ab94c..db2a6a8 100644
--- a/dll/steam_music.h
+++ b/dll/steam_music.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_MUSIC_H
+#define INCLUDED_STEAM_MUSIC_H
+
 class Steam_Music : public ISteamMusic
 {
     int playing;
@@ -41,3 +44,5 @@ public:
 	void SetVolume( float flVolume );
 	float GetVolume();
 };
+
+#endif // INCLUDED_STEAM_MUSIC_H
diff --git a/dll/steam_musicremote.h b/dll/steam_musicremote.h
index 4ad84c2..1cbfe85 100644
--- a/dll/steam_musicremote.h
+++ b/dll/steam_musicremote.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_MUSICREMOTE_H
+#define INCLUDED_STEAM_MUSICREMOTE_H
+
 class Steam_MusicRemote : public ISteamMusicRemote
 {
 public:
@@ -65,3 +68,5 @@ public:
 	bool SetCurrentPlaylistEntry( int nID );
 	bool PlaylistDidChange();
 };
+
+#endif // INCLUDED_STEAM_MUSICREMOTE_H
diff --git a/dll/steam_networking.h b/dll/steam_networking.h
index 4a19845..6b23acc 100644
--- a/dll/steam_networking.h
+++ b/dll/steam_networking.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_NETWORKING_H
+#define INCLUDED_STEAM_NETWORKING_H
+
 //packet timeout in seconds for non connections
 #define ORPHANED_PACKET_TIMEOUT (20)
 #define NEW_CONNECTION_TIMEOUT (20.0)
@@ -1026,3 +1029,5 @@ void Callback(Common_Message *msg)
     }
 }
 };
+
+#endif // INCLUDED_STEAM_NETWORKING_H
diff --git a/dll/steam_networking_messages.h b/dll/steam_networking_messages.h
index 38df49a..ed7a7e8 100644
--- a/dll/steam_networking_messages.h
+++ b/dll/steam_networking_messages.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_NETWORKING_MESSAGES
+#define INCLUDED_STEAM_NETWORKING_MESSAGES
+
 #define NETWORKING_MESSAGES_TIMEOUT 30.0
 
 struct Steam_Message_Connection {
@@ -444,3 +447,5 @@ void Callback(Common_Message *msg)
 }
 
 };
+
+#endif // INCLUDED_STEAM_NETWORKING_MESSAGES
diff --git a/dll/steam_networking_sockets.h b/dll/steam_networking_sockets.h
index 235e7bc..b2f1697 100644
--- a/dll/steam_networking_sockets.h
+++ b/dll/steam_networking_sockets.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_NETWORKING_SOCKETS_H
+#define INCLUDED_STEAM_NETWORKING_SOCKETS_H
+
 struct Listen_Socket {
     HSteamListenSocket socket_id;
 
@@ -2110,3 +2113,5 @@ void Callback(Common_Message *msg)
 }
 
 };
+
+#endif // INCLUDED_STEAM_NETWORKING_SOCKETS_H
diff --git a/dll/steam_networking_socketsserialized.h b/dll/steam_networking_socketsserialized.h
index 8b55292..30b5a2e 100644
--- a/dll/steam_networking_socketsserialized.h
+++ b/dll/steam_networking_socketsserialized.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_NETWORKING_SOCKETS_SERIALIZED_H
+#define INCLUDED_STEAM_NETWORKING_SOCKETS_SERIALIZED_H
+
 class Steam_Networking_Sockets_Serialized :
 public ISteamNetworkingSocketsSerialized002,
 public ISteamNetworkingSocketsSerialized003,
@@ -154,3 +157,5 @@ void Callback(Common_Message *msg)
 }
 
 };
+
+#endif // INCLUDED_STEAM_NETWORKING_SOCKETS_SERIALIZED_H
diff --git a/dll/steam_networking_utils.h b/dll/steam_networking_utils.h
index 0e5388e..6897351 100644
--- a/dll/steam_networking_utils.h
+++ b/dll/steam_networking_utils.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_NETWORKING_UTILS_H
+#define INCLUDED_STEAM_NETWORKING_UTILS_H
+
 class Steam_Networking_Utils :
 public ISteamNetworkingUtils001,
 public ISteamNetworkingUtils002,
@@ -749,3 +752,5 @@ void Callback(Common_Message *msg)
 
 
 };
+
+#endif // INCLUDED_STEAM_NETWORKING_UTILS_H
diff --git a/dll/steam_parental.h b/dll/steam_parental.h
index 5505c61..5048541 100644
--- a/dll/steam_parental.h
+++ b/dll/steam_parental.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_PARENTAL_H
+#define INCLUDED_STEAM_PARENTAL_H
+
 class Steam_Parental : public ISteamParentalSettings
 {
 public:
@@ -29,3 +32,5 @@ public:
 	bool BIsFeatureBlocked( EParentalFeature eFeature );
 	bool BIsFeatureInBlockList( EParentalFeature eFeature );
 };
+
+#endif // INCLUDED_STEAM_PARENTAL_H
diff --git a/dll/steam_parties.h b/dll/steam_parties.h
index 8d282be..4dc1028 100644
--- a/dll/steam_parties.h
+++ b/dll/steam_parties.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_PARTIES_H
+#define INCLUDED_STEAM_PARTIES_H
+
 class Steam_Parties :
 public ISteamParties
 {
@@ -192,3 +195,5 @@ void Callback(Common_Message *msg)
 }
 
 };
+
+#endif // INCLUDED_STEAM_PARTIES_H
diff --git a/dll/steam_remote_storage.h b/dll/steam_remote_storage.h
index 032bc5f..de6bf79 100644
--- a/dll/steam_remote_storage.h
+++ b/dll/steam_remote_storage.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_REMOTE_STORAGE_H
+#define INCLUDED_STEAM_REMOTE_STORAGE_H
+
 struct Async_Read {
 	SteamAPICall_t api_call;
 	uint32 offset;
@@ -791,3 +794,5 @@ bool EndFileWriteBatch()
 
 
 };
+
+#endif // INCLUDED_STEAM_REMOTE_STORAGE_H
diff --git a/dll/steam_remoteplay.h b/dll/steam_remoteplay.h
index 56e2965..c03ea4f 100644
--- a/dll/steam_remoteplay.h
+++ b/dll/steam_remoteplay.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_REMOTEPLAY_H
+#define INCLUDED_STEAM_REMOTEPLAY_H
+
 class Steam_RemotePlay :
 public ISteamRemotePlay
 {
@@ -111,3 +114,5 @@ void Callback(Common_Message *msg)
 }
 
 };
+
+#endif // INCLUDED_STEAM_REMOTEPLAY_H
diff --git a/dll/steam_screenshots.h b/dll/steam_screenshots.h
index 745be9f..9acbe1f 100644
--- a/dll/steam_screenshots.h
+++ b/dll/steam_screenshots.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_SCREENSHOTS_H
+#define INCLUDED_STEAM_SCREENSHOTS_H
+
 struct screenshot_infos_t
 {
 	std::string screenshot_name;
@@ -73,3 +76,5 @@ public:
 	// JPEG, TGA, and PNG formats are supported.
 	ScreenshotHandle AddVRScreenshotToLibrary( EVRScreenshotType eType, const char *pchFilename, const char *pchVRFilename );
 };
+
+#endif // INCLUDED_STEAM_SCREENSHOTS_H
diff --git a/dll/steam_tv.h b/dll/steam_tv.h
index 6ea7857..52738e3 100644
--- a/dll/steam_tv.h
+++ b/dll/steam_tv.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_TV_H
+#define INCLUDED_STEAM_TV_H
+
 class Steam_TV :
 public ISteamTV
 {
@@ -122,3 +125,5 @@ void Callback(Common_Message *msg)
 }
 
 };
+
+#endif // INCLUDED_STEAM_TV_H
diff --git a/dll/steam_ugc.h b/dll/steam_ugc.h
index 77658b2..0b2785c 100644
--- a/dll/steam_ugc.h
+++ b/dll/steam_ugc.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_UGC_H
+#define INCLUDED_STEAM_UGC_H
+
 struct UGC_query {
     UGCQueryHandle_t handle;
     std::set<PublishedFileId_t> return_only;
@@ -903,3 +906,5 @@ SteamAPICall_t GetWorkshopEULAStatus()
 
 
 };
+
+#endif // INCLUDED_STEAM_UGC_H
diff --git a/dll/steam_unified_messages.h b/dll/steam_unified_messages.h
index fb34fb7..b9169a1 100644
--- a/dll/steam_unified_messages.h
+++ b/dll/steam_unified_messages.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_UNIFIED_MESSAGES_H
+#define INCLUDED_STEAM_UNIFIED_MESSAGES_H
+
 class Steam_Unified_Messages :
 public ISteamUnifiedMessages
 {
@@ -121,3 +124,5 @@ void Callback(Common_Message *msg)
 }
 
 };
+
+#endif // INCLUDED_STEAM_UNIFIED_MESSAGES_H
diff --git a/dll/steam_user.h b/dll/steam_user.h
index 0f21124..a316e17 100644
--- a/dll/steam_user.h
+++ b/dll/steam_user.h
@@ -19,6 +19,9 @@
 
 #include "appticket.h"
 
+#ifndef INCLUDED_STEAM_USER_H
+#define INCLUDED_STEAM_USER_H
+
 class Steam_User :
 public ISteamUser009,
 public ISteamUser010,
@@ -532,3 +535,5 @@ bool BSetDurationControlOnlineState( EDurationControlOnlineState eNewState )
 }
 
 };
+
+#endif // INCLUDED_STEAM_USER_H
diff --git a/dll/steam_utils.h b/dll/steam_utils.h
index d229cd3..b473d54 100644
--- a/dll/steam_utils.h
+++ b/dll/steam_utils.h
@@ -19,6 +19,9 @@
 #include "local_storage.h"
 #include "../overlay_experimental/steam_overlay.h"
 
+#ifndef INCLUDED_STEAM_UTILS_H
+#define INCLUDED_STEAM_UTILS_H
+
 static std::chrono::time_point<std::chrono::steady_clock> app_initialized_time = std::chrono::steady_clock::now();
 
 
@@ -442,3 +445,5 @@ bool DismissFloatingGamepadTextInput()
 }
 
 };
+
+#endif // INCLUDED_STEAM_UTILS_H
diff --git a/dll/steam_video.h b/dll/steam_video.h
index 9a1282d..0126ef5 100644
--- a/dll/steam_video.h
+++ b/dll/steam_video.h
@@ -17,6 +17,9 @@
 
 #include "base.h"
 
+#ifndef INCLUDED_STEAM_VIDEO_H
+#define INCLUDED_STEAM_VIDEO_H
+
 class Steam_Video : public ISteamVideo
 {
 public:
@@ -32,3 +35,5 @@ public:
 	void GetOPFSettings( AppId_t unVideoAppID );
 	bool GetOPFStringForApp( AppId_t unVideoAppID, char *pchBuffer, int32 *pnBufferSize );
 };
+
+#endif // INCLUDED_STEAM_VIDEO_H