Skeletal muscle has important metabolic functions but the focus of this article is to examine its ability to generate mechanical force. Adult skeletal muscle fibres are each innervated by a single branch of the axon arising from an α-motoneuron in the spinal cord. The α-motoneuron and all the fibres it innervates constitute a motor unit, and this is the functional unit of the muscle. α-Motoneurons differ in size and excitability and it is the recruitment of these cell bodies in the spinal cord that determines which fibres within the muscle are active during a movement. Correct functioning of the neuromuscular junction is clearly critical for muscle action and it is a site at which many drugs affecting muscle have their action. Here we describe also the mechanism by which skeletal muscle generates force following activation, a process known as excitation–contraction coupling and examine the contractile properties of muscle as well as describing muscle weakness and fatigue and the assessment of muscle performance in health and disease.