Medical information has to be acquired, shared, enhanced and used to result in a coordinated action for individual care of general healthcare. Modelling is a key component of this transformation process. This is not only true at a macroscopic level but also for many sub-domains of medical information engineering. In this chapter, we intend to make clearer the important role of models for Computer Aided Medical Interventions. After having presented a general overview of the needs in terms of modelling, we will focus on two types of models - statistical and biomechanical ones - and we will give examples of their clinical applications. These examples are work in progress developed in collaboration with the Grenoble and Toulouse (Purpan) university hospitals.