The development of MEMS devices differs substantially from product engineering methods used in more traditional industries. The approach is characterized by a close customer involvement and product specific fabrication processes. A large number interdependencies between device design on the one hand and manufacturing process development on the other hand make product engineering in the MEMS area a rather tedious and complicated task. In this paper we discuss a comprehensive customer-oriented MEMS product engineering methodology. Both MEMS design and fabrication process development are analyzed with regard to procedures and interfaces used in order to develop an appropriate CAD support either in terms of existing tools or by specifying individual tools to be implemented. The manufacturing process development is part of this holistic approach and is supported by a CAD environment for the management and the design of thin-film MEMS fabrication processes. This environment has been developed by the authors and became recently commercially available.