Non-shivering thermogenesis, which is required to maintain core body temperature, involves the induction of lipolysis and the expression of thermogenic genes in adipose tissues. Ajay Chawla and colleagues now show a role for interleukin-4 (IL-4) and alternatively activated macrophages in this process. The expression of alternative activation markers was increased in the brown and white adipose tissue (but not in other tissues) of mice acutely exposed to 4°C compared with expression levels in control mice, and this increase depended on IL-4 and IL-13. Cold-exposed mice that lacked alternatively activated macrophages or lacked IL-4 and IL-13 had impaired metabolic adaptations to cold — that is, defects in the induction of lipolysis in white adipose tissue and the upregulation of thermogenic genes in brown adipose tissue. Administration of IL-4 increased non-shivering thermogenesis in a macrophage-dependent manner in wild-type mice housed at normal temperatures. Finally, in response to cold exposure, alternatively activated macrophages from brown and white adipose tissue were shown to produce catecholamines, such as noradrenaline, in an IL-4-dependent manner to mediate the metabolic adaptations to cold.