Body mass and composition reflect the combined effects of three processes: energy intake, energy partitioning (storage), and energy expenditure. Energy is released from food as it is combusted to carbon dioxide and water, and is expended as heat and work within a cell. The energy stores, mainly in adipose tissue, represent the net balance between intake and expenditure. The methods outlined in this unit evaluate these three processes by measuring food intake and lipid absorption, body fat composition, and energy expenditure. Evaluation of food intake and fat mass is a useful first-line phenotyping test indicating altered energy homeostasis. Evaluation of energy expenditure in this unit addresses obligatory basal energy expenditure (for performance of cellular and organ functions), as measured by indirect calorimetry. The combined results of these tests provide indications of the metabolic defects in a mouse model and help to identify molecular targets that cause these abnormalities