A basic concept in object-oriented approaches is the notion of object as integrated unit of structure and behavior. Conceptually, objects are modeled as processes of which certain dynamic characteristics of their internal state can be observed using attributes. Objects are the basic units of design. Systems are composed from objects that interact to provide the desired services. In the semantics domain, concepts related to the object-oriented paradigm like interaction, inheritance and object aggregation can be uniformily modelled by object morphisms.