SummaryThe hypothalamus is one of the oldest and smallest parts of the brain, constituting just 4 gm of the 1400 gm of adult human brain weight. And yet this tiny area contains highly conserved neural circuitry that controls basic life functions: these include energy metabolism, from feeding through digestion, metabolic control, and energy expenditure; fluid and electrolyte balance, from drinking through fluid absorption and excretion; thermoregulation, from choice of environment through heat production and conservation, and fever responses; wake-sleep cycles and emergency responses to stressors in the environment; and reproduction, from reproductive hormone control through mating, pregnancy, birth, and suckling. In this Primer, we will give an overview of the structure of the hypothalamus, and outline what we know about how that relates to its functional circuitry.