With their extremely low mass and volume, low power consumption and tight integration with electronics, MEMS sensors and actuators are extremely appealing for reducing the size and mass of spacecraft without sacrificing functionality. In view of the harsh and remote environment of space, reliability and qualification is the crucial issues that are holding back MEMS from playing a larger role in space applications. We examine how MEMS reliability is handled in commercial MEMS devices used in safety critical applications on earth and contrast the operating conditions on earth with those encountered during launch and in orbit. We explain the impact that vibration, mechanical and thermal shock, and radiation can have on MEMS devices fabricated using the most widespread silicon technologies. Accelerated tests adapted to space qualification are presented as a means to determine the major failure modes. Hermetic packaging is crucial to ensuring long-term reliability.