It is not enough to treat the momentum system as a black box, simply receiving power and control signals and imparting torque or momentum, as if it were any other element in a feedback-control block diagram. A momentum system offers unique opportunities to achieve robust and lightweight spacecraft designs, but taking advantage of these opportunities requires careful attention to rigid- and flexible-body dynamics. This chapter provides the foundational equations of motion for spacecraft with momentum-control devices and summarizes important flexible effects that drive the design of spacecraft that incorporate these actuators.