This text discusses the applications, challenges, future, and limitations of MEMS for aerospace and aeronautical applications. It begins with an overview of the capabilities of MEMS, the systems that can be generated, and some fundamentals on material properties and physical phenomena on the small scale. A discussion of the varied applications in aerospace follows, including micro-machined or MEMS-based spacecraft propulsion, MEMS turbines, Sterling engines, and louvers for power and thermal control, and MEMS flaps, jets, or bubbles for control of an air foil. The editors conclude with a review of reliability concerns, stressing such limiting risks as friction, fatigue, and electrostatic stiction.