This contribution describes electrical activation of the neuromuscular system with electrodes positioned in or on skeletal muscle. The mechanisms of neuromuscular stimulation are described, with reference to transmission at the neuromuscular junction, to illustrate that it is the motor nerve, rather than the muscle, that is stimulated. The two methods to modulate muscle force, recruitment and rate modulation, are reviewed with reference to their basis in the properties of the neuromuscular system including the force-frequency relationship of skeletal muscle and the strength-duration, current-distance, and current-diameter relationships for excitation of motor nerve fibers. The elements that impact the shape of the input-output (recruitment) curve of muscle force as a function of stimulation intensity are presented, including a description of position-dependent recruitment. In addition, the biomechanical impacts on motor output including the length-tension properties of skeletal muscle and the transformation of muscle force to joint moment are reviewed. This contribution serves as a comprehensive review of the fundamental aspects of neuromuscular stimulation.