Current software development tools let developers model a software system and generate program code from the models to run the system. However, generating code and installing a non-trivial system induces a time delay between changing the model and executing it that makes rapid model prototyping awkard if not impossible. This paper presents the architecture of a virtual machine for UML that interprets UML models without any intermediate code-generation step. The paper shows how to embed UML in a metalevel architecture so that a key property of model-based systems, the casual connection between models and model instances, is guaranteed. With this architecture, changes to a model have immediate effects on its execution, providing users with rapid feedback about the model's structure and behavior. This approach supports model innovation better than today's code-generation approaches.