Model-based testing relies on models of a system under test and/or its environment to derive test cases for the system. This paper provides an overview of the field. Seven different dimensions define a taxonomy that allows the characterization of different approaches to model-based testing. It is intended to help with understanding benefits and limitations of model-based testing, understanding the approach used in a particular model-based testing tool, and understanding the issues involved in integrating model-based testing into a software development process. To illustrate the taxonomy, we classify several approaches embedded in existing model-based testing tools.