This chapter introduces readers to basic work physiology including muscles, structure and function, and capacity, aerobic capacity, muscle fatigue, endurance, and recovery from work demands. It describes the length–tension relationship. Work capacity is defined together with the personal and environmental factors that affect work capacity. The concept of fatigue is discussed in detail as well as the stress response and the function of the sympathetic and parasympathetic nervous systems. A method for estimating endurance times for static muscles exertions is described with a worked example. A method for estimating oxygen consumption from heart rate is also given. Tools for the assessment of physical work demands are described including tools for ambulatory monitoring such as heart rate monitors and pedometers. A method for estimating rest periods for manual workers is described with a worked example. Scales and checklists for evaluating work demands are given including the Borg scale and the need for recovery from work scale.