diff --git a/src/game/server/ai_scriptconditions.cpp b/src/game/server/ai_scriptconditions.cpp
index 7e637ba5190..d1fdf4c22ce 100644
--- a/src/game/server/ai_scriptconditions.cpp
+++ b/src/game/server/ai_scriptconditions.cpp
@@ -14,10 +14,17 @@
 #include "ai_scriptconditions.h"
 #include "saverestore_utlvector.h"
 
+#ifdef MAPBASE_MP
+#include "filters.h"
+#endif
+
 // memdbgon must be the last include file in a .cpp file!!!
 #include "tier0/memdbgon.h"
 
 #define SF_ACTOR_AS_ACTIVATOR	( 1 << 0 )
+#ifdef MAPBASE
+#define SF_PLAYER_AS_ACTIVATOR	( 1 << 1 )	// Mainly useful in MP
+#endif
 
 ConVar debugscriptconditions( "ai_debugscriptconditions", "0" );
 
@@ -55,6 +62,9 @@ BEGIN_DATADESC( CAI_ScriptConditions )
 #ifdef MAPBASE
 	DEFINE_INPUTFUNC( FIELD_EHANDLE, "SatisfyConditions", InputSatisfyConditions ),
 #endif
+#ifdef MAPBASE_MP
+	DEFINE_INPUTFUNC( FIELD_STRING, "SetPlayerFilter", InputSetPlayerFilter ),
+#endif
 
 	//---------------------------------
 
@@ -111,6 +121,11 @@ BEGIN_DATADESC( CAI_ScriptConditions )
 	DEFINE_UTLVECTOR( m_ElementList,				FIELD_EMBEDDED ),
 	DEFINE_FIELD( m_bLeaveAsleep,					FIELD_BOOLEAN ),
 
+#ifdef MAPBASE_MP
+	DEFINE_KEYFIELD( m_iszPlayerFilterName,				FIELD_STRING,	"PlayerFilter"			),
+	DEFINE_FIELD( m_hPlayerFilter, FIELD_EHANDLE ),
+#endif
+
 END_DATADESC()
 
 BEGIN_SIMPLE_DATADESC( CAI_ProxTester )
@@ -129,6 +144,42 @@ END_DATADESC()
 
 #define EVALUATOR( name ) { &CAI_ScriptConditions::Eval##name, #name }
 
+#ifdef MAPBASE_MP
+
+CAI_ScriptConditions::EvaluatorInfo_t CAI_ScriptConditions::gm_PlayerEvaluators[] =
+{
+	EVALUATOR( ActorSeePlayer ),
+		EVALUATOR( PlayerActorProximity ),
+		EVALUATOR( PlayerTargetProximity ),
+		EVALUATOR( PlayerBlockingActor ),
+		EVALUATOR( PlayerActorLook ),
+		EVALUATOR( PlayerTargetLook ),
+		EVALUATOR( PlayerActorLOS ),
+		EVALUATOR( PlayerTargetLOS ),
+
+#ifdef HL2_EPISODIC
+		EVALUATOR( PlayerInVehicle ),
+#endif
+
+};
+
+// !!! In MP, this is only used for evaluators that don't use players.
+// Use gm_PlayerEvaluators for evaluators that require a player.
+CAI_ScriptConditions::EvaluatorInfo_t CAI_ScriptConditions::gm_Evaluators[] =
+{
+		EVALUATOR( State ),
+		EVALUATOR( ActorTargetProximity ),
+		EVALUATOR( ActorSeeTarget),
+
+#ifdef HL2_EPISODIC
+		EVALUATOR( ActorInPVS ),
+		EVALUATOR( ActorInVehicle ),
+#endif
+
+};
+
+#else
+
 CAI_ScriptConditions::EvaluatorInfo_t CAI_ScriptConditions::gm_Evaluators[] =
 {
 	EVALUATOR( ActorSeePlayer ),
@@ -151,6 +202,8 @@ CAI_ScriptConditions::EvaluatorInfo_t CAI_ScriptConditions::gm_Evaluators[] =
 
 };
 
+#endif
+
 void CAI_ScriptConditions::OnRestore( void )
 {
 	BaseClass::OnRestore();
@@ -537,7 +590,7 @@ void CAI_ScriptConditions::EvaluationThink()
 			m_OnConditionsTimeout.FireOutput( pActivator, this );
 			continue;
 		}
-
+		
 		bool      result = true;
 		const int nEvaluators = sizeof( gm_Evaluators ) / sizeof( gm_Evaluators[0] );
 
@@ -561,6 +614,47 @@ void CAI_ScriptConditions::EvaluationThink()
 			}
 		}
 
+#ifdef MAPBASE_MP
+		if ( result )
+		{
+			// Loop through all of the players until one of them passes. If we've been given a filter, use it to determine which players to evaluate.
+			// 
+			// You may notice this ignores GetPlayer() and is implemented more primitively than how ai_script_conditions handles multiple actors.
+			// ai_script_conditions was designed to evaluate conditions on multiple actors simultaneously, but it was only designed with one player in mind.
+			// Having the existing element list account for multiple players is theoretically possible, but each element is designed to time out and fire outputs separately.
+			// Since existing instances of the entity were not designed for that functionality and it would make the entity much more complex to manage even under new cases,
+			// I've elected to go for a simpler implementation that just loops through all of the players every time an actor's condition needs to be evaluated.
+			const int nPlayerEvaluators = sizeof( gm_PlayerEvaluators ) / sizeof( gm_PlayerEvaluators[0] );
+			for ( int playerIdx = 1; playerIdx <= gpGlobals->maxClients; playerIdx++ )
+			{
+				CBasePlayer *pPlayer = UTIL_PlayerByIndex( playerIdx );
+				if ( !pPlayer || ( m_hPlayerFilter && !m_hPlayerFilter->PassesFilter( this, pPlayer ) ) )
+					continue;
+
+				result = true;
+				args.pPlayer = pPlayer;
+
+				ScrCondDbgMsg( ("%s evaluating player: %s [%i]\n", GetDebugName(), pPlayer->GetPlayerName(), playerIdx) );
+
+				for ( int i = 0; i < nPlayerEvaluators; ++i )
+				{
+					if ( !(this->*gm_PlayerEvaluators[i].pfnEvaluator)( args ) )
+					{
+						pConditionElement->GetTimer()->Reset();
+						result = false;
+
+						ScrCondDbgMsg( ( "\t%s failed on: %s\n", GetDebugName(), gm_PlayerEvaluators[ i ].pszName ) );
+
+						break;
+					}
+				}
+
+				if ( result )
+					break;
+			}
+		}
+#endif
+
 		if ( result )
 		{
 			ScrCondDbgMsg( ( "%s waiting... %f\n", GetDebugName(), pConditionElement->GetTimer()->GetRemaining() ) );
@@ -656,6 +750,13 @@ void CAI_ScriptConditions::Enable( void )
 		}
 	}
 
+#ifdef MAPBASE_MP
+	if ( m_iszPlayerFilterName != NULL_STRING )
+	{
+		m_hPlayerFilter = dynamic_cast<CBaseFilter *>(gEntList.FindEntityByName( NULL, m_iszPlayerFilterName, this ));
+	}
+#endif
+
 	m_fDisabled = false;
 
 	SetThink( &CAI_ScriptConditions::EvaluationThink );
@@ -706,6 +807,23 @@ void CAI_ScriptConditions::InputSatisfyConditions( inputdata_t &inputdata )
 }
 #endif
 
+#ifdef MAPBASE_MP
+//-----------------------------------------------------------------------------
+
+void CAI_ScriptConditions::InputSetPlayerFilter( inputdata_t &inputdata )
+{
+	m_iszPlayerFilterName = inputdata.value.StringID();
+	if ( m_iszPlayerFilterName != NULL_STRING )
+	{
+		m_hPlayerFilter = dynamic_cast<CBaseFilter *>(gEntList.FindEntityByName( NULL, m_iszPlayerFilterName, this ));
+	}
+	else
+	{
+		m_hPlayerFilter = NULL;
+	}
+}
+#endif
+
 //-----------------------------------------------------------------------------
 
 bool CAI_ScriptConditions::IsInFOV( CBaseEntity *pViewer, CBaseEntity *pViewed, float fov, bool bTrueCone )
diff --git a/src/game/server/ai_scriptconditions.h b/src/game/server/ai_scriptconditions.h
index 10e5929bac2..25418c0d172 100644
--- a/src/game/server/ai_scriptconditions.h
+++ b/src/game/server/ai_scriptconditions.h
@@ -162,6 +162,9 @@ class CAI_ScriptConditions : public CBaseEntity, public IEntityListener
 #ifdef MAPBASE
 	void InputSatisfyConditions( inputdata_t &inputdata );
 #endif
+#ifdef MAPBASE_MP
+	void InputSetPlayerFilter( inputdata_t &inputdata );
+#endif
 
 	// Output handlers
 	COutputEvent	m_OnConditionsSatisfied;
@@ -186,7 +189,10 @@ class CAI_ScriptConditions : public CBaseEntity, public IEntityListener
 		EvaluationFunc_t	pfnEvaluator;
 		const char			*pszName;
 	};
-
+	
+#ifdef MAPBASE_MP
+	static EvaluatorInfo_t gm_PlayerEvaluators[];
+#endif
 	static EvaluatorInfo_t gm_Evaluators[];
 
 	//---------------------------------
@@ -247,6 +253,11 @@ class CAI_ScriptConditions : public CBaseEntity, public IEntityListener
 	ThreeState_t	m_fActorInVehicle;
 	ThreeState_t	m_fPlayerInVehicle;
 
+#ifdef MAPBASE_MP
+	string_t				m_iszPlayerFilterName;
+	CHandle<CBaseFilter> 	m_hPlayerFilter;
+#endif
+
 	CUtlVector< CAI_ScriptConditionsElement > m_ElementList;
 
 	//---------------------------------
diff --git a/src/game/server/env_player_surface_trigger.cpp b/src/game/server/env_player_surface_trigger.cpp
index e25038ec383..7b3a250c320 100644
--- a/src/game/server/env_player_surface_trigger.cpp
+++ b/src/game/server/env_player_surface_trigger.cpp
@@ -15,7 +15,13 @@ LINK_ENTITY_TO_CLASS( env_player_surface_trigger, CEnvPlayerSurfaceTrigger );
 
 BEGIN_DATADESC( CEnvPlayerSurfaceTrigger )
 	DEFINE_KEYFIELD( m_iTargetGameMaterial, FIELD_INTEGER, "gamematerial" ),
+#ifdef MAPBASE_MP
+	DEFINE_AUTO_ARRAY( m_iCurrentGameMaterial, FIELD_INTEGER ),
+	DEFINE_AUTO_ARRAY( m_iLastGameMaterial, FIELD_INTEGER ),
+	DEFINE_FIELD( m_nNumOnMaterial, FIELD_INTEGER ),
+#else
 	DEFINE_FIELD( m_iCurrentGameMaterial, FIELD_INTEGER ),
+#endif
 	DEFINE_FIELD( m_bDisabled, FIELD_BOOLEAN ),
 
 	DEFINE_THINKFUNC( UpdateMaterialThink ),
@@ -27,6 +33,11 @@ BEGIN_DATADESC( CEnvPlayerSurfaceTrigger )
 	// Outputs
 	DEFINE_OUTPUT(m_OnSurfaceChangedToTarget, "OnSurfaceChangedToTarget"),
 	DEFINE_OUTPUT(m_OnSurfaceChangedFromTarget, "OnSurfaceChangedFromTarget"),
+#ifdef MAPBASE
+	// Used in MP
+	DEFINE_OUTPUT( m_OnSurfaceChangedToTargetAll, "OnSurfaceChangedToTargetAll" ),
+	DEFINE_OUTPUT( m_OnSurfaceChangedFromTargetAll, "OnSurfaceChangedFromTargetAll" ),
+#endif
 END_DATADESC()
 
 // Global list of surface triggers
@@ -48,7 +59,15 @@ void CEnvPlayerSurfaceTrigger::Spawn( void )
 	SetSolid( SOLID_NONE );
 	SetMoveType( MOVETYPE_NONE );
 
+#ifdef MAPBASE_MP
+	for (int i = 0; i < MAX_PLAYERS; i++)
+	{
+		m_iCurrentGameMaterial[i] = 0;
+		m_iLastGameMaterial[i] = 0;
+	}
+#else
 	m_iCurrentGameMaterial = 0;
+#endif
 	m_bDisabled = false;
 
 	g_PlayerSurfaceTriggers.AddToTail( this );
@@ -90,6 +109,19 @@ void CEnvPlayerSurfaceTrigger::PlayerSurfaceChanged( CBasePlayer *pPlayer, char
 		return;
 
 	// Fire the output if we've changed, but only if it involves the target material
+#ifdef MAPBASE_MP
+	int idx = pPlayer->entindex();
+	if ( gameMaterial != (char)(m_iCurrentGameMaterial[idx]) &&
+	     ( gameMaterial == m_iTargetGameMaterial || m_iCurrentGameMaterial[idx] == m_iTargetGameMaterial ) )
+	{
+		DevMsg( 2, "Player changed material to %d (was %d)\n", gameMaterial, m_iCurrentGameMaterial[idx] );
+
+		m_iCurrentGameMaterial[idx] = (int)gameMaterial;
+
+		SetThink( &CEnvPlayerSurfaceTrigger::UpdateMaterialThink );
+		SetNextThink( gpGlobals->curtime );
+	}
+#else
 	if ( gameMaterial != (char)m_iCurrentGameMaterial &&
 	     ( gameMaterial == m_iTargetGameMaterial || m_iCurrentGameMaterial == m_iTargetGameMaterial ) )
 	{
@@ -100,6 +132,7 @@ void CEnvPlayerSurfaceTrigger::PlayerSurfaceChanged( CBasePlayer *pPlayer, char
 		SetThink( &CEnvPlayerSurfaceTrigger::UpdateMaterialThink );
 		SetNextThink( gpGlobals->curtime );
 	}
+#endif
 }
 
 //-----------------------------------------------------------------------------
@@ -108,14 +141,63 @@ void CEnvPlayerSurfaceTrigger::PlayerSurfaceChanged( CBasePlayer *pPlayer, char
 //-----------------------------------------------------------------------------
 void CEnvPlayerSurfaceTrigger::UpdateMaterialThink( void )
 {
+#ifdef MAPBASE_MP
+	CBasePlayer *pFirstPlayer = NULL;
+	int nNumOnMaterialLast = m_nNumOnMaterial;
+
+	for (int i = 0; i < gpGlobals->maxClients; i++)
+	{
+		CBasePlayer *pPlayer = UTIL_PlayerByIndex( i );
+		if (!pFirstPlayer)
+			pFirstPlayer = pPlayer;
+
+		if ( m_iCurrentGameMaterial[i] != m_iLastGameMaterial[i] )
+		{
+			if ( m_iCurrentGameMaterial[i] == m_iTargetGameMaterial )
+			{
+				m_OnSurfaceChangedToTarget.FireOutput( pPlayer, this );
+				m_nNumOnMaterial++;
+			}
+			else 
+			{
+				m_OnSurfaceChangedFromTarget.FireOutput( pPlayer, this );
+
+				if (m_nNumOnMaterial > 0)
+					m_nNumOnMaterial--;
+			}
+		}
+
+		m_iLastGameMaterial[i] = m_iCurrentGameMaterial[i];
+	}
+
+	if ( nNumOnMaterialLast == 0 && m_nNumOnMaterial > 0 )
+	{
+		m_OnSurfaceChangedToTargetAll.FireOutput( pFirstPlayer, this );
+	}
+	else if ( nNumOnMaterialLast > 0 && m_nNumOnMaterial == 0 )
+	{
+		m_OnSurfaceChangedFromTargetAll.FireOutput( pFirstPlayer, this );
+	}
+#else
 	if ( m_iCurrentGameMaterial == m_iTargetGameMaterial )
 	{
 		m_OnSurfaceChangedToTarget.FireOutput( NULL, this );
+
+#ifdef MAPBASE
+		// This is used in MP, but SP has only one player, so just fire it here
+		m_OnSurfaceChangedToTargetAll.FireOutput( NULL, this );
+#endif
 	}
 	else 
 	{
 		m_OnSurfaceChangedFromTarget.FireOutput( NULL, this );
+
+#ifdef MAPBASE
+		// This is used in MP, but SP has only one player, so just fire it here
+		m_OnSurfaceChangedFromTargetAll.FireOutput( NULL, this );
+#endif
 	}
+#endif
 }
 
 //-----------------------------------------------------------------------------
@@ -124,6 +206,9 @@ void CEnvPlayerSurfaceTrigger::UpdateMaterialThink( void )
 void CEnvPlayerSurfaceTrigger::InputDisable( inputdata_t &inputdata )
 {
 	m_bDisabled = true;
+#ifdef MAPBASE_MP
+	m_nNumOnMaterial = 0;
+#endif
 }
 
 //-----------------------------------------------------------------------------
diff --git a/src/game/server/env_player_surface_trigger.h b/src/game/server/env_player_surface_trigger.h
index 96eb0810d9c..0eae16d6534 100644
--- a/src/game/server/env_player_surface_trigger.h
+++ b/src/game/server/env_player_surface_trigger.h
@@ -38,12 +38,22 @@ class CEnvPlayerSurfaceTrigger : public CPointEntity
 
 private:
 	int		m_iTargetGameMaterial;
+#ifdef MAPBASE_MP
+	int		m_iCurrentGameMaterial[MAX_PLAYERS];
+	int		m_iLastGameMaterial[MAX_PLAYERS];
+	int		m_nNumOnMaterial;
+#else
 	int		m_iCurrentGameMaterial;
+#endif
 	bool	m_bDisabled;
 
 	// Outputs
 	COutputEvent m_OnSurfaceChangedToTarget;
 	COutputEvent m_OnSurfaceChangedFromTarget;
+#ifdef MAPBASE
+	COutputEvent m_OnSurfaceChangedToTargetAll;
+	COutputEvent m_OnSurfaceChangedFromTargetAll;
+#endif
 };
 
 #endif // ENV_PLAYER_SURFACE_TRIGGER_H