Biological entities obey physical laws, and, in this regard, humans and other mammals obey the laws of thermodynamics. Body energy stores can only increase and obesity can only occur when food intake exceeds energy expenditure (or metabolic rate). Similarly, energy stores can only be depleted when energy expenditure exceeds food intake. Thus, the balance between food intake and energy expenditure determines the body's energy stores. The quantity of energy stored by the human body is impressive; lean individuals store 2–3 months of their energy needs in adipose tissue whereas obese persons can carry a year's worth of their energy needs. The cumulative impact of energy imbalance over months and years can result in the development of obesity. The factors that regulate appetite and food intake are discussed elsewhere. In this chapter we will discuss the importance of physical activity as a component of energy expenditure.