To investigate the mechanical behavior and failure modes of micro-electromechanical systems (MEMS), a high resolution optical tool capable of imaging both fast and slow movements in three dimensions on a microscopic scale is almost mandatory. We report the first experiments with a new tool, which provides the necessary speed and resolution. First, some common failure modes in MEMS are discussed, as are the possibilities of optical inspection methods to deal with them. We show that the system we describe may be used to monitor the resonances and small but fast movements of a MEMS device, to image erratic behavior, stuck parts, and so on. Furthermore, slow deformations, for example of expansion due to temperature changes, can be monitored with high resolution, and give information on deformation or delamination of layers.