diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7b06bf3..b461f10 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,26 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Added
+
+- Added `implementProperty` and `implementMethod` to the `roblox` built-in library to fill in missing functionality that Lune does not aim to implement itself.
+
+  Example usage:
+
+  ```lua
+  local roblox = require("@lune/roblox")
+
+  local part = roblox.Instance.new("Part")
+
+  roblox.implementMethod("BasePart", "TestMethod", function(_, ...)
+      print("Tried to call TestMethod with", ...)
+  end)
+
+  part:TestMethod("Hello", "world!")
+  ```
+
 ## `0.7.8` - October 5th, 2023
 
 ### Added
diff --git a/src/tests.rs b/src/tests.rs
index 89ef6dc..c4162fd 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -164,6 +164,10 @@ create_tests! {
     roblox_instance_classes_workspace: "roblox/instance/classes/Workspace",
     roblox_instance_classes_terrain: "roblox/instance/classes/Terrain",
 
+    roblox_instance_custom_async: "roblox/instance/custom/async",
+    roblox_instance_custom_methods: "roblox/instance/custom/methods",
+    roblox_instance_custom_properties: "roblox/instance/custom/properties",
+
     roblox_instance_methods_clear_all_children: "roblox/instance/methods/ClearAllChildren",
     roblox_instance_methods_clone: "roblox/instance/methods/Clone",
     roblox_instance_methods_destroy: "roblox/instance/methods/Destroy",
diff --git a/tests/roblox/instance/custom/async.luau b/tests/roblox/instance/custom/async.luau
new file mode 100644
index 0000000..284567f
--- /dev/null
+++ b/tests/roblox/instance/custom/async.luau
@@ -0,0 +1,12 @@
+local roblox = require("@lune/roblox")
+
+local game = roblox.Instance.new("DataModel")
+local http = game:GetService("HttpService") :: any
+
+roblox.implementMethod("HttpService", "GetAsync", function()
+	-- TODO: Fill in method body
+end)
+
+-- TODO: Fill in rest of test cases here
+
+http:GetAsync()
diff --git a/tests/roblox/instance/custom/methods.luau b/tests/roblox/instance/custom/methods.luau
new file mode 100644
index 0000000..0797093
--- /dev/null
+++ b/tests/roblox/instance/custom/methods.luau
@@ -0,0 +1,48 @@
+local roblox = require("@lune/roblox")
+
+local inst = roblox.Instance.new("Instance") :: any
+local part = roblox.Instance.new("Part") :: any
+
+-- Basic sanity checks for callbacks
+
+local success = pcall(function()
+	inst:Wat()
+end)
+assert(not success, "Nonexistent methods should error")
+
+roblox.implementMethod("Instance", "Wat", function() end)
+
+local success2 = pcall(function()
+	inst:Wat()
+end)
+assert(success2, "Nonexistent methods should error, unless implemented")
+
+-- Instance should be passed to callback
+
+roblox.implementMethod("Instance", "PassingInstanceTest", function(instance)
+	assert(instance == inst, "Invalid instance was passed to callback")
+end)
+roblox.implementMethod("Part", "PassingPartTest", function(instance)
+	assert(instance == part, "Invalid instance was passed to callback")
+end)
+inst:PassingInstanceTest()
+part:PassingPartTest()
+
+-- Any number of args passed & returned should work
+
+roblox.implementMethod("Instance", "Echo", function(_, ...)
+	return ...
+end)
+
+local one, two, three = inst:Echo("one", "two", "three")
+assert(one == "one", "implementMethod callback should return proper values")
+assert(two == "two", "implementMethod callback should return proper values")
+assert(three == "three", "implementMethod callback should return proper values")
+
+-- Methods implemented by Lune should take precedence
+
+roblox.implementMethod("Instance", "FindFirstChild", function()
+	error("unreachable")
+end)
+inst:FindFirstChild("Test")
+part:FindFirstChild("Test")
diff --git a/tests/roblox/instance/custom/properties.luau b/tests/roblox/instance/custom/properties.luau
new file mode 100644
index 0000000..01ad508
--- /dev/null
+++ b/tests/roblox/instance/custom/properties.luau
@@ -0,0 +1,64 @@
+local roblox = require("@lune/roblox")
+
+local inst = roblox.Instance.new("Instance") :: any
+local part = roblox.Instance.new("Part") :: any
+
+-- Basic sanity checks for callbacks
+
+local success = pcall(function()
+	local _ = inst.Wat
+end)
+assert(not success, "Nonexistent properties should error")
+
+roblox.implementProperty("Instance", "Wat", function()
+	return nil
+end)
+
+local success2 = pcall(function()
+	local _ = inst.Wat
+end)
+assert(success2, "Nonexistent properties should error, unless implemented")
+
+-- Instance should be passed to callback
+
+roblox.implementProperty("Instance", "PassingInstanceTest", function(instance)
+	assert(instance == inst, "Invalid instance was passed to callback")
+	return nil
+end)
+roblox.implementProperty("Part", "PassingPartTest", function(instance)
+	assert(instance == part, "Invalid instance was passed to callback")
+	return nil
+end)
+local _ = inst.PassingInstanceTest
+local _ = part.PassingPartTest
+
+-- Any number of args passed & returned should work
+
+local counters = {}
+roblox.implementProperty("Instance", "Counter", function(instance)
+	-- FIXME: Instances do not make for unique table keys for some reason ...
+	local value = counters[tostring(instance)] or 0
+	value += 1
+	counters[tostring(instance)] = value
+	return value
+end, function(instance, value)
+	counters[tostring(instance)] = value
+end)
+
+assert(inst.Counter == 1, "implementProperty callback should return proper values")
+assert(inst.Counter == 2, "implementProperty callback should return proper values")
+assert(inst.Counter == 3, "implementProperty callback should return proper values")
+
+inst.Counter = 10
+
+assert(inst.Counter == 11, "implementProperty callback should set proper values")
+assert(inst.Counter == 12, "implementProperty callback should return proper values")
+assert(inst.Counter == 13, "implementProperty callback should return proper values")
+
+-- Properties implemented by Lune should take precedence
+
+roblox.implementProperty("Instance", "Parent", function()
+	error("unreachable")
+end)
+local _ = inst.Parent
+local _ = part.Parent
diff --git a/types/roblox.luau b/types/roblox.luau
index 880f060..6a769db 100644
--- a/types/roblox.luau
+++ b/types/roblox.luau
@@ -391,6 +391,113 @@ function roblox.getReflectionDatabase(): Database
 	return nil :: any
 end
 
+--[=[
+	@within Roblox
+
+	Implements a property for all instances of the given `className`.
+
+	This takes into account class hierarchies, so implementing a property
+	for the `BasePart` class will also implement it for `Part` and others,
+	unless a more specific implementation is added to the `Part` class directly.
+
+	### Behavior
+
+	The given `getter` callback will be called each time the property is
+	indexed, with the instance as its one and only argument. The `setter`
+	callback, if given, will be called each time the property should be set,
+	with the instance as the first argument and the property value as second.
+
+	### Example usage
+
+	```lua
+	local roblox = require("@lune/roblox")
+
+	local part = roblox.Instance.new("Part")
+
+	local propertyValues = {}
+	roblox.implementProperty(
+		"BasePart",
+		"CoolProp",
+		function(instance)
+			if propertyValues[instance] == nil then
+				propertyValues[instance] = 0
+			end
+			propertyValues[instance] += 1
+			return propertyValues[instance]
+		end,
+		function(instance, value)
+			propertyValues[instance] = value
+		end
+	)
+
+	print(part.CoolProp) --> 1
+	print(part.CoolProp) --> 2
+	print(part.CoolProp) --> 3
+
+	part.CoolProp = 10
+
+	print(part.CoolProp) --> 11
+	print(part.CoolProp) --> 12
+	print(part.CoolProp) --> 13
+	```
+
+	@param className The class to implement the property for.
+	@param propertyName The name of the property to implement.
+	@param getter The function which will be called to get the property value when indexed.
+	@param setter The function which will be called to set the property value when indexed. Defaults to a function that will error with a message saying the property is read-only.
+]=]
+function roblox.implementProperty<T>(
+	className: string,
+	propertyName: string,
+	getter: (instance: Instance) -> T,
+	setter: ((instance: Instance, value: T) -> ())?
+)
+	return nil :: any
+end
+
+--[=[
+	@within Roblox
+
+	Implements a method for all instances of the given `className`.
+
+	This takes into account class hierarchies, so implementing a method
+	for the `BasePart` class will also implement it for `Part` and others,
+	unless a more specific implementation is added to the `Part` class directly.
+
+	### Behavior
+
+	The given `callback` will be called every time the method is called,
+	and will receive the instance it was called on as its first argument.
+	The remaining arguments will be what the caller passed to the method, and
+	all values returned from the callback will then be returned to the caller.
+
+	### Example usage
+
+	```lua
+	local roblox = require("@lune/roblox")
+
+	local part = roblox.Instance.new("Part")
+
+	roblox.implementMethod("BasePart", "TestMethod", function(instance, ...)
+	    print("Called TestMethod on instance", instance, "with", ...)
+	end)
+
+	part:TestMethod("Hello", "world!")
+	--> Called TestMethod on instance Part with Hello, world!
+	```
+
+	@param className The class to implement the method for.
+	@param methodName The name of the method to implement.
+	@param callback The function which will be called when the method is called.
+]=]
+function roblox.implementMethod(
+	className: string,
+	methodName: string,
+	callback: (instance: Instance, ...any) -> ...any
+)
+	return nil :: any
+end
+
 -- TODO: Make typedefs for all of the datatypes as well...
 roblox.Instance = (nil :: any) :: {
 	new: ((className: "DataModel") -> DataModel) & ((className: string) -> Instance),