The Unity runtime is written in C/C++. This runtime is used in any build you create using the editor - for webplayers and plugins it is installed separate from your build, whereas it is included in it for stand-alones and other platforms such as iPhone and Wii.

The editor is built on the Unity runtime and additionally includes editor-specific C/C++ binaries.

Wrapped around the Unity core is a layer which allows for .net access to core functionality. This layer is used for user scripting and for most of the editor UI.

Unity is written in C++, with the following exceptions:

• we expose a .NET api so that you don't have to go trough the pain of writing your game in c++, but you can write it in Javascript or c# or boo.

• The editor application's UI is written in C#, using mostly the same API that we expose to game developers. (We have access to a few API's that we don't expose (yet), but not that many)

This is also the reason that the answer to the question "Can I use UnityEngine.dll in some other c# project" is always no. There is hardly any functionality in UnityEngine.dll, the only thing it does is relay your c#/javascript calls into the C++ part of Unity. Without the C++ part there is nothing.

This can definitely be answered in more detail by one of the UT engineers, but as far as I know, it's a mixture of both. Certainly a lot of the core code is in c++, including the physics engine and sound engine, however as far as I understand, much (if not all) of the Editor UI itself, including the terrain engine and its editing tools, is written in c# using Unity's own editor scripting.