Model-driven engineering is a powerful approach to build large-scale applications. However, an application’s metamodel often remains static after the initial development phase and cannot be changed unless a new development effort occurs. Yet, end users often need to rapidly adapt their applications to new needs. In many cases, end users would know how to make the required adaptations, if only the application would let them do so. In this paper we present how we built a runtimedynamic meta-environment into Smalltalk’s reflective language model. Our solution offers the best of both worlds: developers can develop their applications using the same tools they are used to and gain the power of meta-programming. We show in particular that our approach is suitable to support end user customization without writing new code: the adaptive model of Magritte not only describes existing classes, but also lets end users build their own metamodels on the fly.