A review of MEMS (micro electro mechanical systems) and MOMS (micro opto mechanical systems) is presented, highlighting comparisons and contrasts in applying the respective technologies from a complete system viewpoint. Typical examples from each technology are described, including an outline of the principles of operation and important assessment criteria in choosing an appropriate technology for a particular application. Assessment factors include performance requirements such as accuracy and resolution in addition to media compatibility, of which operating temperatures/pressures, chemical attack and electromagnetic immunity are important considerations.