diff --git a/code/controllers/subsystem/mobs.dm b/code/controllers/subsystem/mobs.dm
index 1311ca1589..bcdb1af8ed 100644
--- a/code/controllers/subsystem/mobs.dm
+++ b/code/controllers/subsystem/mobs.dm
@@ -1,14 +1,18 @@
 SUBSYSTEM_DEF(mobs)
 	name = "Mobs"
 	priority = 100
-	flags = SS_KEEP_TIMING|SS_NO_INIT
+	flags = SS_KEEP_TIMING
 	runlevels = RUNLEVEL_GAME | RUNLEVEL_POSTGAME
 
 	var/list/currentrun = list()
+	var/static/list/clients_by_zlevel[][]
 
 /datum/controller/subsystem/mobs/stat_entry()
 	..("P:[GLOB.mob_living_list.len]")
 
+/datum/controller/subsystem/mobs/Initialize(start_timeofday)
+	clients_by_zlevel = new /list(world.maxz,0)
+	return ..()
 
 /datum/controller/subsystem/mobs/fire(resumed = 0)
 	var/seconds = wait * 0.1
diff --git a/code/game/atoms_movable.dm b/code/game/atoms_movable.dm
index 4d7a417e53..3e0f0112ff 100644
--- a/code/game/atoms_movable.dm
+++ b/code/game/atoms_movable.dm
@@ -254,6 +254,12 @@
 		loc = destination
 
 		if(!same_loc)
+			var/turf/oldturf = get_turf(oldloc)
+			var/turf/destturf = get_turf(destination)
+			var/old_z = (oldturf ? oldturf.z : null)
+			var/dest_z = (destturf ? destturf.z : null)
+			if (old_z != dest_z)
+				onTransitZ(old_z, dest_z)
 			destination.Entered(src, oldloc)
 			if(destarea && old_area != destarea)
 				destarea.Entered(src, oldloc)
@@ -276,6 +282,11 @@
 				old_area.Exited(src, null)
 		loc = null
 
+/atom/movable/proc/onTransitZ(old_z,new_z)
+	for (var/item in src) // Notify contents of Z-transition. This can be overridden IF we know the items contents do not care.
+		var/atom/movable/AM = item
+		AM.onTransitZ(old_z,new_z)
+
 //Called whenever an object moves and by mobs when they attempt to move themselves through space
 //And when an object or action applies a force on src, see newtonian_move() below
 //Return 0 to have src start/keep drifting in a no-grav area and 1 to stop/not start drifting
diff --git a/code/game/machinery/vending.dm b/code/game/machinery/vending.dm
index 90b1327b9e..bfa89db368 100644
--- a/code/game/machinery/vending.dm
+++ b/code/game/machinery/vending.dm
@@ -1190,6 +1190,9 @@ IF YOU MODIFY THE PRODUCTS LIST OF A MACHINE, MAKE SURE TO UPDATE ITS RESUPPLY C
 	refill_canister = /obj/item/vending_refill/games
 
 
+/obj/machinery/vending/onTransitZ()
+	return
+
 #undef STANDARD_CHARGE
 #undef CONTRABAND_CHARGE
 #undef COIN_CHARGE
\ No newline at end of file
diff --git a/code/game/turfs/space/space.dm b/code/game/turfs/space/space.dm
index 54c43cdcea..1d7b32e3a2 100644
--- a/code/game/turfs/space/space.dm
+++ b/code/game/turfs/space/space.dm
@@ -130,9 +130,12 @@
 		return
 
 	if(destination_z)
+		var/old_z = A.z
 		A.x = destination_x
 		A.y = destination_y
 		A.z = destination_z
+		if (old_z != destination_z)
+			A.onTransitZ(old_z, destination_z)
 
 		if(isliving(A))
 			var/mob/living/L = A
diff --git a/code/modules/lighting/lighting_object.dm b/code/modules/lighting/lighting_object.dm
index 23d6bdba50..fe25dc9439 100644
--- a/code/modules/lighting/lighting_object.dm
+++ b/code/modules/lighting/lighting_object.dm
@@ -140,6 +140,9 @@
 /atom/movable/lighting_object/blob_act()
 	return
 
+/atom/movable/lighting_object/onTransitZ()
+	return
+
 // Override here to prevent things accidentally moving around overlays.
 /atom/movable/lighting_object/forceMove(atom/destination, var/no_tp=FALSE, var/harderforce = FALSE)
 	if(harderforce)
diff --git a/code/modules/mining/satchel_ore_boxdm.dm b/code/modules/mining/satchel_ore_boxdm.dm
index 4c772a7019..8e58da8e05 100644
--- a/code/modules/mining/satchel_ore_boxdm.dm
+++ b/code/modules/mining/satchel_ore_boxdm.dm
@@ -81,3 +81,6 @@
 		WD.add_fingerprint(user)
 	dump_box_contents()
 	qdel(src)
+
+/obj/structure/ore_box/onTransitZ()
+	return
\ No newline at end of file
diff --git a/code/modules/mob/living/carbon/human/interactive.dm b/code/modules/mob/living/carbon/human/interactive.dm
index 35e83a8e04..9fd2cbf8de 100644
--- a/code/modules/mob/living/carbon/human/interactive.dm
+++ b/code/modules/mob/living/carbon/human/interactive.dm
@@ -828,7 +828,7 @@
 		return pick(/area/hallway, /area/crew_quarters/locker)
 
 /mob/living/carbon/human/interactive/proc/target_filter(target)
-	var/list/filtered_targets = list(/area, /turf, /obj/machinery/door, /atom/movable/light, /obj/structure/cable, /obj/machinery/atmospherics)
+	var/list/filtered_targets = list(/area, /turf, /obj/machinery/door, /atom/movable/lighting_object, /obj/structure/cable, /obj/machinery/atmospherics)
 	var/list/L = target
 	for(var/atom/A in target) // added a bunch of "junk" that clogs up the general find procs
 		if(is_type_in_list(A,filtered_targets))
diff --git a/code/modules/mob/living/life.dm b/code/modules/mob/living/life.dm
index 1f0895a7f5..0aafef753f 100644
--- a/code/modules/mob/living/life.dm
+++ b/code/modules/mob/living/life.dm
@@ -8,6 +8,18 @@
 	if((movement_type & FLYING) && !floating)	//TODO: Better floating
 		float(on = TRUE)
 
+	if (client || registered_z) // This is a temporary error tracker to make sure we've caught everything
+		var/turf/T = get_turf(src)
+		if (client && registered_z != T.z)
+#ifdef TESTING
+			message_admins("[src] [ADMIN_FLW(src)] has somehow ended up in Z-level [T.z] despite being registered in Z-level [registered_z]. If you could ask them how that happened and notify coderbus, it would be appreciated.")
+#endif
+			log_game("Z-TRACKING: [src] has somehow ended up in Z-level [T.z] despite being registered in Z-level [registered_z].")
+			update_z(T.z)
+		else if (!client && registered_z)
+			log_game("Z-TRACKING: [src] of type [src.type] has a Z-registration despite not having a client.")
+			update_z(null)
+
 	if (notransform)
 		return
 	if(!loc)
diff --git a/code/modules/mob/living/living.dm b/code/modules/mob/living/living.dm
index 3ca234434c..f589ecbb20 100644
--- a/code/modules/mob/living/living.dm
+++ b/code/modules/mob/living/living.dm
@@ -1039,4 +1039,19 @@
 	if(.)
 		if(client)
 			reset_perspective(destination)
-		update_canmove() //if the mob was asleep inside a container and then got forceMoved out we need to make them fall.
\ No newline at end of file
+		update_canmove() //if the mob was asleep inside a container and then got forceMoved out we need to make them fall.
+
+/mob/living/proc/update_z(new_z) // 1+ to register, null to unregister
+	if (registered_z != new_z)
+		if (registered_z)
+			SSmobs.clients_by_zlevel[registered_z] -= src
+		if (client)
+			if (new_z)
+				SSmobs.clients_by_zlevel[new_z] += src
+			registered_z = new_z
+		else
+			registered_z = null
+
+/mob/living/onTransitZ(old_z,new_z)
+	..()
+	update_z(new_z)
\ No newline at end of file
diff --git a/code/modules/mob/living/living_defines.dm b/code/modules/mob/living/living_defines.dm
index 3b5ae247df..50f2ce4db0 100644
--- a/code/modules/mob/living/living_defines.dm
+++ b/code/modules/mob/living/living_defines.dm
@@ -77,4 +77,6 @@
 
 	var/last_words	//used for database logging
 
-	var/list/obj/effect/proc_holder/abilities = list()
\ No newline at end of file
+	var/list/obj/effect/proc_holder/abilities = list()
+
+	var/registered_z
\ No newline at end of file
diff --git a/code/modules/mob/living/login.dm b/code/modules/mob/living/login.dm
index 97ffa72aec..ffdd37bdca 100644
--- a/code/modules/mob/living/login.dm
+++ b/code/modules/mob/living/login.dm
@@ -13,6 +13,10 @@
 	update_damage_hud()
 	update_health_hud()
 
+	if (loc)
+		var/turf/T = get_turf(src)
+		update_z(T.z)
+
 	//Vents
 	if(ventcrawler)
 		to_chat(src, "<span class='notice'>You can ventcrawl! Use alt+click on vents to quickly travel about the station.</span>")
diff --git a/code/modules/mob/living/logout.dm b/code/modules/mob/living/logout.dm
index a3479aac89..9b998b2d89 100644
--- a/code/modules/mob/living/logout.dm
+++ b/code/modules/mob/living/logout.dm
@@ -1,6 +1,14 @@
+<<<<<<< HEAD
 /mob/living/Logout()
 	if(ranged_ability && client)
 		ranged_ability.remove_mousepointer(client)
 	..()
+=======
+/mob/living/Logout()
+	update_z(null)
+	if(ranged_ability && client)
+		ranged_ability.remove_mousepointer(client)
+	..()
+>>>>>>> de51ac3... Player Z-tracking, again (#33216)
 	if(!key && mind)	//key and mind have become separated.
 		mind.active = 0	//This is to stop say, a mind.transfer_to call on a corpse causing a ghost to re-enter its body.
\ No newline at end of file
diff --git a/code/modules/shuttle/on_move.dm b/code/modules/shuttle/on_move.dm
index 15f0dbb011..c7a77f9457 100644
--- a/code/modules/shuttle/on_move.dm
+++ b/code/modules/shuttle/on_move.dm
@@ -89,15 +89,23 @@ All ShuttleMove procs go here
 			return
 
 	loc = newT
+
 	return TRUE
 
 // Called on atoms after everything has been moved
 /atom/movable/proc/afterShuttleMove(turf/oldT, list/movement_force, shuttle_dir, shuttle_preferred_direction, move_dir, rotation)
+
+	var/turf/newT = get_turf(src)
+	if (newT.z != oldT.z)
+		onTransitZ(oldT.z, newT.z)
+
 	if(light)
 		update_light()
 	if(rotation)
 		shuttleRotate(rotation)
 
+
+
 	update_parallax_contents()
 
 	return TRUE
@@ -328,9 +336,6 @@ All ShuttleMove procs go here
 /atom/movable/lighting_object/onShuttleMove()
 	return FALSE
 
-/atom/movable/light/onShuttleMove()
-	return FALSE
-
 /obj/docking_port/stationary/onShuttleMove(turf/newT, turf/oldT, list/movement_force, move_dir, obj/docking_port/stationary/old_dock, obj/docking_port/mobile/moving_dock)
 	if(!moving_dock.can_move_docking_ports || old_dock == src)
 		return FALSE