This paper presents an interaction model adapted to mixed reality environments known as IRVO (Interacting with Real and Virtual Objects). IRVO aims at modeling the interaction between one or more users and the Mixed Reality system by representing explicitly the objects and tools involved and their relationship. IRVO covers the design phase of the life cycle and models the intended use of the system. In a first part, we present a brief review of related HCI models. The second part is devoted to the IRVO model, its notation and some examples. In the third part, we present how IRVO is used for designing applications and in particular we show how this model can be integrated in a Model-Based Approach (CoCSys) which is currently designed at our lab.