diff --git a/src/ecs.lua b/src/ecs.lua
index 14db8fd..bb21600 100644
--- a/src/ecs.lua
+++ b/src/ecs.lua
@@ -1,10 +1,29 @@
-local ENTITY_STORAGE = {}
-local PLACEHOLDER = {}
-
 ecs = { }
 
+local ALL_ENTITIES = {}
+
+---@alias System { callback: fun(delta: number, a: any, b: any, c: any, d: any, e: any, any), shapes: {}, keys: string[], entityCache: table<any, boolean> }
+
+---@type System[]
+local SYSTEMS = {}
+
+local PLACEHOLDER = {}
+
+-- TODO: Add entity to any existing systems
 function ecs.addEntity(entity)
-	ENTITY_STORAGE[entity] = true
+	ALL_ENTITIES[entity] = true
+	for _, system in pairs(SYSTEMS) do
+		if entityMatchesShapes(entity, system.shapes) then
+		    system.entityCache[entity] = true
+		end
+	end
+end
+
+function ecs.removeEntity(entity)
+	ALL_ENTITIES[entity] = nil
+	for _, system in pairs(SYSTEMS) do
+	    system.entityCache[entity] = nil
+	end
 end
 
 ---@return any
@@ -21,49 +40,15 @@ function allKeysIncluded(entity, filter)
 	return true
 end
 
-function allFiltersMatch(entity, filters)
-	for _, filter in pairs(filters) do
-		if not allKeysIncluded(entity, filter) then
+function entityMatchesShapes(entity, shapes)
+	for _, shape in pairs(shapes) do
+		if not allKeysIncluded(entity, shape) then
 		    return false
 		end
 	end
 	return true
 end
 
----@generic T
----@param shape T
----@return fun(): T[]
-function ecs.buildEntityFilter(shape)
-	local filters = {shape}
-	return function()
-		-- TODO? Cache all this?
-		local ret = {}
-		for entity,_ in pairs(ENTITY_STORAGE) do
-			if allFiltersMatch(entity, filters) then
-				ret[#ret + 1] = entity;
-			end
-		end
-		return ret
-	end
-end
-
----@generic T
----@return fun(): T[]
-function ecs.buildRequiredFields(...)
-	local filters = arg
-	return function()
-		-- TODO? Cache all this?
-		local ret = {}
-		for entity,_ in pairs(ENTITY_STORAGE) do
-			if allKeysIncluded(entity, filters) then
-				ret[#ret + 1] = entity;
-			end
-		end
-		return ret
-	end
-end
--- --@overload fun(tShape: `T`): fun(callback: fun(componentT: `T`))
-
 ---@generic T
 ---@generic U
 ---@generic V
@@ -73,11 +58,55 @@ end
 ---@param vShape V?
 ---@param wShape W?
 ---@return fun(callback: fun(componentT: T, componentU: U, componentV: V, componentW: W))
-function ecs.entitiesWithShapes(tShape, uShape, vShape, wShape)
+function ecs.entitiesHavingShapes(tShape, uShape, vShape, wShape)
 	return function()
 	end
 end
 
+-- Print contents of `tbl`, with indentation.
+-- `indent` sets the initial level of indentation.
+function tprint (tbl, indent)
+  if not indent then indent = 0 end
+  for k, v in pairs(tbl) do
+    formatting = string.rep("  ", indent) .. k .. ": "
+    if type(v) == "table" then
+      print(formatting)
+      tprint(v, indent+1)
+    elseif type(v) == 'boolean' then
+      print(formatting .. tostring(v))
+    else
+      print(formatting .. v)
+    end
+  end
+end
+
+function addSystem(callback, keys, shapes)
+	SYSTEMS[#SYSTEMS + 1] = {
+		callback = callback,
+		keys = keys,
+		shapes = shapes,
+		entityCache = nil
+	}
+end
+
+---@param deltaSeconds number
+function ecs.update(deltaSeconds)
+	for _,system in pairs(SYSTEMS) do
+		if not system.entityCache then
+		    system.entityCache = {}
+			for entity,_ in pairs(ALL_ENTITIES) do
+				if entityMatchesShapes(entity, system.shapes) then
+					system.entityCache[entity] = true
+				end
+			end
+		end
+		for entity,_ in pairs(system.entityCache) do
+			local keys = system.keys
+			system.callback(deltaSeconds, entity[keys[1]], entity[keys[2]], entity[keys[3]], entity[keys[4]], entity)
+		end
+	end
+end
+
 --- Returns a function that accepts a callback. This callback will receive one argument for each Shape provided.
 ---
 ---@generic T
@@ -89,82 +118,51 @@ end
 ---@generic W
 ---@generic WKey
 ---@param tShape { [TKey]: T }
----@param uShape { [UKey]: U }?
----@param vShape { [VKey]: V }?
----@param wShape { [WKey]: W }?
----@return fun(callback: fun(componentT: T, componentU: U, componentV: V, componentW: W))
-function ecs.entitiesWithShapes2(tShape, uShape, vShape, wShape)
-	local filters = {tShape, uShape, vShape, wShape}
-	return function(callback)
-		-- TODO? Cache all this?
-		local ret = {}
-		for entity,_ in pairs(ENTITY_STORAGE) do
-			if allFiltersMatch(entity, filters) then
-				callback(entity, entity, entity, entity)
-			end
+---@param uShape { [UKey]: U } | fun(componentT: T, any) | nil
+---@param vShape { [VKey]: V } | fun(componentT: T, componentU: U, any) | nil
+---@param wShape { [WKey]: W } | fun(componentT: T, componentU: U, componentV: V, any) | nil
+---@param finalFunc fun(componentT: T, componentU: U, componentV: V, componentW: W, any) | nil
+function ecs.entitiesWithShapes(tShape, uShape, vShape, wShape, finalFunc)
+	local maybeShapes = {tShape, uShape, vShape, wShape, finalFunc}
+	local shapes = {}
+	local callback
+
+	for _,maybeShape in pairs(maybeShapes) do
+		if type(maybeShape) == "table" then
+		    shapes[#shapes + 1] = maybeShape
+		elseif type(maybeShape) == "function" then
+			callback = maybeShape
 		end
-		return ret
 	end
+
+	local keys = {}
+	for _,shape in pairs(shapes) do
+		for key,_ in pairs(shape) do
+			keys[#keys + 1] = key
+		end
+	end
+
+	addSystem(callback, keys, shapes)
 end
 
 local f = ecs.field()
 
----@alias XYPair { x:number, y: number }
-
----@type XYPair
+---@type { x: number, y: number }
 local XYPair = { x = f, y = f }
 
-local HasPosition = { position = XYPair }
-local HasVelocity = { velocity = XYPair }
+local Position = { position = XYPair }
+local Velocity = { velocity = XYPair }
 
-local entIt = ecs.entitiesWithShapes(HasPosition, HasVelocity)
-entIt(function(hasPos, hasVel)
-	hasPos.position.x = hasPos.position.x + hasVel.velocity.y
-end)
-
-local entIt2 = ecs.entitiesWithShapes2(HasPosition)
-entIt2(function(position, velocity)
-	---@type number
-	local num = 0
-	num = position.x
-	num = velocity.z
-	print(num)
-end)
-
----@return fun(callback: fun(T))
-function ecs.entitiesWithFields(...)
-	local filters = {...}
-	return function(callback)
-		-- TODO? Cache all this?
-		for entity,_ in pairs(ENTITY_STORAGE) do
-			if allKeysIncluded(entity, filters) then
-				callback(entity)
-			end
-		end
-	end
-end
-
----@type unknown
-function ecs.shape(...)
-	local shape = {}
-	for _,incomingShape in ipairs(arg) do
-		for k,v in pairs(incomingShape) do
-			shape[k] = v
-		end
-	end
-	return shape
-end
-
-local exampleEntity = {
+local someMovable = {
 	position = { x = 0, y = 0 },
-	velocity = { x = 1, y = 1 }
+	velocity = { x = 2, y = 1 }
 }
 
-ecs.addEntity(exampleEntity)
+ecs.addEntity(someMovable)
 
----@type fun(system: fun(entity: { position: XYPair, velocity: XYPair }))
-local forEachPosVel = ecs.entitiesWithFields('position', 'velocity')
-
-forEachPosVel(function(hasVelPos)
-	print(hasVelPos.velocity.x)
+ecs.entitiesWithShapes(Position, Velocity, function (delta, pos, vel)
+	print("position:")
+	tprint(pos, 1)
+	pos.x = pos.x + (vel.x * delta)
+	pos.y = pos.y + (vel.y * delta)
 end)
\ No newline at end of file