A new language called Modelica for hierarchical physical modeling is developed in an international effort. The main objectives are to make it easy to exchange models and model libraries between tools and to use object-oriented constructs to facilitate the reuse of modeling knowledge. The design approach joins the concepts of non-causal modeling with ordinary differential and algebraic equations with object-oriented model structuring facilities. Modelica's language features are compared to the features of Smile, one of the existing object-oriented modeling languages available from the TU of Berlin and GMD FIRST. The Smile environment is used at DLR Cologne for the modeling of solar thermal power plants, but the Smile development team plans to extend the environment with the Modelica language (Ernst, T.,S. Jahnichen, M. Klose 1997). The example of a simple stove with electrical plates is used to point out the new model structuring possibilities. A tank system with a centrifugal pump is u..