Energy is defined as the capacity for doing work. It is the heat produced in the body, which is utilized for performing the involuntary and voluntary activities, to maintain body temperature to synthesize new body constituents. This chapter covers the units of energy, energy requirements of different categories of people, components of energy requirements, measurement of energy, body mass index and basic metabolism, basal metabolic rate, and determinants and factors affecting. Energy requirement is the amount of food energy needed to balance energy expenditure in order to maintain body size, body composition and a level of necessary and desirable physical activity consistent with long-term good health. Body mass index is a simple index of weight-for-height that is commonly used to classify underweight, overweight and obesity in adults.