Previous works define the notion of system model, which provides a formal basis for the definition of the semantics of a distributed object-oriented modelling language. This article addresses the UML 2.0 interactions and their meaning in terms of a system model. One denotational and two operational approaches are discussed. These are based on existing proposals for the semantics of interactions, but in principle alternative proposals can also be employed. Keywords. UML, interactions, denotational semantics, operational semantics, system model