This edition of 'Reliability of MEMS' was originally published in the successful series 'Advanced Micro & Nanosystems'. Here, one of the most important hurdles to commercialization for microelectromechanical systems is covered in detail: the reliability of MEMS materials and devices. Due to their microscale size combined with novel functionalities, a whole new category of challenges arises, and proper determination of a given device's reliability is instrumental in determining its range of usability and application fields. Any kind of gadget's performance, lifetime and safety will depend on the continued and predictable functioning of both the electronic as well as the micromechanical parts. MEMS reliability therefore can be as serious as human life-and-death matters – quite literally in the case of roll-over sensors for cars, for example.