Virtually all spacecraft incorporate some form of momentum control. Even the simplest spin-stabilized spacecraft—nothing more than a rigid body stabilized by virtue of its own spin—might be considered to benefit from a form of momentum control. Dual-spin spacecraft incorporate a spinning “rotor” and a non-spinning “platform,” which achieves attitude stabilization through a similar but more versatile approach. Contemporary spacecraft use a wide range of momentum-control technologies—momentum wheels, reaction wheels, and several types of control-moment gyroscopes—with an ever-increasing number of applications. The applications discussed here focus on contemporary spacecraft, from small autonomous satellites to large human-space vehicles and earth-observation systems. However, the hardware and basic principles have found uses outside the field of aerospace engineering, largely in commercial applications. Examples include stabilization of trains, automobiles, and boats. As spinoffs from the high-tech, high-reliability world of aerospace make their way into other fields, one can expect to find reaction wheels and CMGs in even more surprising places.