In this paper, we present an approach to define the semantics for object-oriented modeling languages. One important property of this semantics is to support underspecified and incomplete models. To this end, semantics is given as predicates over elements of the semantic domain. This domain is called the system model which is a general declarative characterization of object systems. The system model is very detailed since it captures various relevant structural, behavioral, and interaction aspects. This allows us to re-use the system model as a domain for various kinds of object-oriented modeling languages. As a major consequence the integration of language semantics is straight-forward. The whole approach is supported by tools that do not constrain the semantics definition's expressiveness and flexibility while making it machinecheckable.