A new methodology for the object-oriented description of models consisting of a mixture of continuous and discrete components is presented. The object-oriented paradigm enables the user to describe such models in a modular fashion that permits the reuse of these models independently of the environment in which they are to be embedded. The paper explains the basic mechanisms needed for object-oriented modeling of hybrid systems by means of language constructs available in the object-oriented modeling language Dymola. It then addresses more advanced concepts such as variable structure models containing e.g. ideal electrical switches, ideal diodes and dry friction.