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BMR refers to minimal energy expenditure for essential functions when a person is awake, resting, thermoneutral, and post-absorptive after an overnight fast. It accounts for a substantial share of daily energy use in sedentary adults, while activity and other processes contribute the rest. Indirect calorimetry estimates energy expenditure from oxygen consumption and carbon dioxide production; predictive equations provide another approach. Lean body mass is a major influence, alongside factors such as age, sex, hormones, illness, fever, pregnancy, and nutritional state. The lesson also distinguishes BMR from resting metabolic rate and introduces respiratory quotient as a clue to substrate use. Understanding these concepts supports estimates of energy needs in weight management and clinical nutrition.