Behaviour trees is a formalism mostly used by the game industry to model the behaviour of non-player characters. Since it is used by independent communities, it lacks consistency in the definition of the model and almost no metrics are provided. This report addresses these issues by presenting a well defined formalism for the behaviour trees and environment and non-environment oriented metrics. Moreover, the metrics were tested inside a game simulation. It is important to note that for running the simulation you will need Unity 3.5.1.