This paper presents an overview of the Possum specification animation system, an addition to the Cogito Methodology and Toolset. Possum allows interpretation (or animation) of specifications written in Sum, which is the specification language used in Cogito. We give an account of the functionality of Possum, illustrated by some simple examples, and describe the way in which Possum is used in a typical Cogito development. The current capabilities and limitations of Possum are reviewed from a technical perspective and an overview of other systems that support the animation of formal specification languages is presented.