diff --git a/_en/quality/luacheck.md b/_en/quality/luacheck.md
index fee2e5b..a734b61 100644
--- a/_en/quality/luacheck.md
+++ b/_en/quality/luacheck.md
@@ -18,7 +18,7 @@ editor to provide alerts to any mistakes.
 * [Running LuaCheck](#running-luacheck)
 * [Configuring LuaCheck](#configuring-luacheck)
     * [Troubleshooting](#troubleshooting)
-* [Checking commits with Travis](#checking-commits-with-travis)
+* [Checking Commits with Travis](#checking-commits-with-travis)
 
 ## Installing LuaCheck
 
@@ -103,7 +103,7 @@ available.
 * **Atom** - `linter-luacheck`
 * **Sublime** - `SublimeLinter-luacheck`
 
-## Checking commits with Travis
+## Checking Commits with Travis
 
 If your project is public and is on Github, you can use TravisCI - a free service
 to run jobs on commits to check them. This means that every commit you push will
diff --git a/_en/quality/unit_testing.md b/_en/quality/unit_testing.md
index 46e5d4e..b8006eb 100644
--- a/_en/quality/unit_testing.md
+++ b/_en/quality/unit_testing.md
@@ -7,34 +7,185 @@ idx: 6.4
 
 ## Introduction
 
-Unit tests are an essential tool in proving and resuring yourself that your code
+Unit tests are an essential tool in proving and reassuring yourself that your code
 is correct. This chapter will show you how to write tests for Minetest mods and
-games using busted, and how to structure your code to make this easier - Writing
-unit tests for functions where you call Minetest functions is quite difficult,
-but luckily [in the previous chapter](mvc.html), we discussed how to make your
-code avoid this.
+games using busted. Writing unit tests for functions where you call Minetest
+functions is quite difficult, but luckily [in the previous chapter](clean_arch.html)
+ we discussed how to make your code avoid this.
 
 * [Installing Busted](#installing-busted)
-    * [Windows](#windows)
-    * [Linux](#linux)
-
+* [Your First Test](#your-first-test)
+* [Mocking: Using External Functions](#mocking-using-external-functions)
+* [Checking Commits with Travis](#checking-commits-with-travis)
+* [Conclusion](#conclusion)
 
 ## Installing Busted
 
-### Windows
+First you'll need to install LuaRocks.
 
-*Todo. No one cares about windows, right?*
+* Windows: Follow the [installation instructions on LuaRock's wiki](https://github.com/luarocks/luarocks/wiki/Installation-instructions-for-Windows).
+* Debian/Ubuntu Linux: `sudo apt install luarocks`
 
-### Linux
-
-First you'll need to install LuaRocks:
-
-    sudo apt install luarocks
-
-You can then install Busted globally:
+Next you should then install Busted globally:
 
     sudo luarocks install busted
 
-Check that it's installed with the following command:
+Finally, check that it is installed:
 
     busted --version
+
+
+## Your First Test
+
+Busted is Lua's leading unit test framework. Busted looks for Lua files with
+names ending in `_spec`, and then executes them in a standalone Lua environment.
+
+    mymod/
+    ├── init.lua
+    ├── api.lua
+    └── tests
+        └── api_spec.lua
+
+
+### init.lua
+
+{% highlight lua %}
+mymod = {}
+
+dofile(minetest.get_modpath("mymod") .. "/api.lua")
+{% endhighlight %}
+
+
+
+### api.lua
+
+{% highlight lua %}
+function mymod.add(x, y)
+    return x + y
+end
+{% endhighlight %}
+
+### tests/api_spec.lua
+
+{% highlight lua %}
+-- Look for required things in
+package.path = "../?.lua;" .. package.path
+
+-- Set mymod global for API to write into
+_G.mymod = {} --_
+-- Run api.lua file
+require("api")
+
+-- Tests
+describe("add", function()
+    it("adds", function()
+        assert.equals(2, mymod.add(1, 1))
+    end)
+
+    it("supports negatives", function()
+        assert.equals(0,  mymod.add(-1,  1))    
+        assert.equals(-2, mymod.add(-1, -1))
+    end)
+end)
+{% endhighlight %}
+
+You can now run the tests by opening a terminal in the mod's directory and
+running `busted .`
+
+It's important that the api file doesn't create the table itself, as globals in
+Busted work differently. Any variable which would be global in Minetest is instead
+a file local in busted. This would have been a better way for Minetest to do things,
+but it's too late for that now.
+
+Another thing to note is that any files you're testing should avoid calls to any
+functions not inside of it. You tend to only write tests for a single file at once.
+
+
+## Mocking: Using External Functions
+
+Mocking is the practice of replacing functions that the thing you're testing depends
+on. This can have two purposes - firstly, the function may not be available in the
+test environment. Secondly, you may want to capture calls to the function and any
+passed arguments.
+
+If you follow the advice in the [Clean Architectures](clean_arch.html) chapter,
+you'll already have a pretty clean file to test. You will still have to mock
+things not in your area however - for example, you'll have to mock the view when
+testing the controller/API. If you didn't follow the advice, then things are a
+little harder as you may have to mock the Minetest API.
+
+{% highlight lua %}
+-- As above, make a table
+_G.minetest = {}
+
+-- Define the mock function
+local chat_send_all_calls = {}
+function minetest.chat_send_all(name, message)
+    table.insert(chat_send_all_calls, { name = name, message = message })
+end
+
+-- Tests
+describe("list_areas", function()
+    it("returns a line for each area", function()
+        chat_send_all_calls = {} -- reset table
+
+        mymod.list_areas_to_chat("singleplayer", "singleplayer")
+
+        assert.equals(2, #chat_send_all_calls)
+    end)
+
+    it("sends to right player", function()
+        chat_send_all_calls = {} -- reset table
+
+        mymod.list_areas_to_chat("singleplayer", "singleplayer")
+
+        for _, call in pairs(chat_send_all_calls) do --_
+            assert.equals("singleplayer", call.name)
+        end
+    end)
+
+    -- The above two tests are actually pointless, as this one tests both things
+    it("returns correct thing", function()
+        chat_send_all_calls = {} -- reset table
+
+        mymod.list_areas_to_chat("singleplayer", "singleplayer")
+
+        local expected = {
+            { name = "singleplayer", message = "Town Hall (2,43,63)" },
+            { name = "singleplayer", message = "Airport (43,45,63)" },
+        }
+        assert.same(expected, chat_send_all_calls)
+    end)
+end)
+{% endhighlight %}
+
+
+## Checking Commits with Travis
+
+The Travis script from the [Error Checking](luacheck.html)
+chapter can be modified to also run Busted.
+
+{% highlight yml %}
+language: generic
+sudo: false
+addons:
+  apt:
+    packages:
+    - luarocks
+before_install:
+  - luarocks install --local luacheck && luarocks install --local busted
+script:
+- $HOME/.luarocks/bin/luacheck --no-color .
+- $HOME/.luarocks/bin/busted .
+notifications:
+  email: false
+{% endhighlight %}
+
+
+## Conclusion
+
+Unit tests will greatly increase the quality and reliability of your project if used
+well, but they require you to structure your code in a different way than usual.
+
+For an example of a mod with lots of unit tests, see
+[crafting by rubenwardy](https://github.com/rubenwardy/crafting).