Our approach for modelling interactive systems has been to develop models for the interface and interaction which are lightweight but with an underlying formal semantics. Combined with traditional formal methods to describe functional behaviour, this provides the ability to create a single formal model of interactive systems and consider all parts (functionality, user interface and interaction) with the same rigorous level of formality. The ability to convert the different models we use from one notation to another has given us a set of models which describe an interactive system (or parts of that system) at different levels of abstraction in ways most suitable for the domain but which can be combined into a single model for model checking, theorem proving, etc. There are, however, many benefits to using the individual models for different purposes throughout the development process. In this chapter, we provide examples of this using the nuclear power plant control system as an example.