diff --git a/doc/Concepts.md b/doc/Concepts.md
index eac4d04..92eaf5a 100644
--- a/doc/Concepts.md
+++ b/doc/Concepts.md
@@ -81,6 +81,31 @@ ctx.setForegroundColor(White);
 ctx.write("Hello world!");
 ```
 
+## Under the hood
+
+There are a number of interfaces and classes that needs to be explained to understand how the GUI works.
+
+`TermWriteable` is an interface that allows the usage of the normal CC terminal write methods. Stuff like `write`, `setCursorPos` and `setCursorBlink` are defined here. This is of course implemented by the physical screens and the main terminal.
+
+Most of the time you will not write directory to a real screen but to a `VirtualTermWriter` which extends `TermWriteable` with some more methods like `enable`
+and `setTarget`. The `setTarget` is used as the proxy target of a `VirtualTermWriter` and with `enable` and `disable` you can enable and disable the forwarding of the write methods to the target.
+
+The `StatelessVirtualTermWriter` and `BufferedVirtualTermWriter` are both `VirtualTermWriter`. They can have a real output as a target. Or they can have another `VirtualTermWriter` as target like the `BufferedVirtualTermWriter` which uses a `TermBuffer` as an intermediate target.
+
+All of that is just for printing to the screen. If you want to read input you have to use the `WindowContext` which is a `TermWriteable`. 
+`WindowContext` also handles events like `onClick` or `onKey`. This is need so that the right program gets the input depending on the active window on the 
+screen or terminal.
+
+All of the `WindowContext` are managed by the `WindowManager`. The `WindowManager` also delegates the events to the right `WindowContext`.
+
+## GUI helper classes
+
+Because we want a more abstract way of writing to the screen we have some "helper" classes. I call them "helper" but they a very essential to the GUI.
+
+First there is the `Pixel` class which is nothing more that a char and a foreground and background color. 
+
+A collection of `Pixel` is called a `Canvas` which is nothing more than a 2D array of `Pixel` with some functions strapped to it.
+
 # Proceses
 
 The concept of processes tryes to encapsulate programs. A process is basically an interface with the `run(handle: ProcessHandle)` method.