This book is a result of the European framework project muFly, aiming at the design and realization of an autonomous micro helicopter comparable to a small bird in size and mass. It introduces two prototype designs for the muFly helicopter: a modular one for fast sensor and actuator exchange and easy configuration testing, and a highly integrated design to reduce the total mass and integrate the complete sensor set for autonomous flight. Furthermore, a theoretical foundation for coaxial micro helicopter design is laid. In particular focus are passive stabilization and steering principles of the helicopter. A modular dynamic model is developed, which incorporates the characteristics of a hingeless rotor system, two optional steering principles, and a stabilizer bar module, which is also validated experimentally. Finally, the dynamic model is used for a design parameter study of the most relevant system design parameters. It aims at passive roll and pitch stabilization of the helicopter without a stabilizer bar. By illuminating the aspects of prototype design, dynamic modeling, simulation, and design parameter optimization, this book is a contribution towards palm-sized MAVs.