This section describes the fundamentals, operating principles, and dynamical modeling of the actuators used for active spacecraft attitude control. Magnetic torquers and thrusters are presented as a way of applying external control torques to a spacecraft to alter both attitude and momentum. Momentum exchange devices, consisting of reaction and momentum wheels and control moment gyros are then presented as means of generating internal control torques for the operational attitude control of a spacecraft. The evolution of space applications is putting increasingly stringent requirements on satellite attitude control performance, thence the increasing importance of attitude control hardware.