Over the last few years, considerable effort has gone into the study of the failure mechanisms and reliability of micro-electromechanical systems (MEMS). Although still very incomplete, our knowledge of the reliability issues relevant to MEMS is growing. This paper provides an overview of MEMS failure mechanisms that are commonly encountered. It focuses on the reliability issues of micro-scale devices, but, for some issues, the field of their macroscopic counterparts is also briefly touched. The paper discusses generic structures used in MEMS, stiction, creep, fatigue, brittle fatigue in silicon, wear, dielectric charging, breakdown, contamination and packaging.