How to create Multi Project SBT Build Files

A while back I started programming Scala for the first time. It was also the first introduction to the scala build tool. The defacto standard build to for compiling and packaging scala applications.

The first steps were easy, I had projects up and running in no time. But now I've come across a scenario where I need to create a piece of software that actually is composed of multiple projects. This presented me with an interesting problem. How can you build a project with SBT that depends on other projects that also use SBT.