We measured meteorological conditions and estimated the energy costs of thermoregulation for young and adult Adélie Penguins (Pygoscelis adeliae) at a breeding colony near the Antarctic Peninsula. Air temperatures averaged < 5°C and strong winds were frequent. Operative temperatures (T_e) for adults ranged from -8 to 28°C, averaging 5-6°C, for the period from courtship to fledging of chicks. The average energy cost of thermoregulation (C_th) for adult penguins was equivalent to 10-16% of basal metabolism. C_th comprised about 15% of the estimated daily energy budget (DEB) of incubating adults, but only about 1% of the DEB of adults feeding chicks. The T_e's for chicks older than 14 days ranged from 0 to 31°C, averaging 8.0 C. The C_th for downy chicks ranged from about 31% of minimal metabolic rate (MMR) in 1 kg chicks to about 10% of MMR in 3 kg chicks. Between initial thermal independence (age 12-14 days) and the cessation of parental feeding (age 35-40 days), chicks use about 10-11% of assimilated energy for thermoregulation. C_th is equivalent to about 17% of the MMR of fledglings during their 2-3 week fast. We observed no indication of thermal stress (i.e., conditions in which birds cannot maintain stable T_b) in adults and no indication of cold stress in any age class. However, on clear, calm days when air temperature exceeds 7-10°C for several hours, downy chicks are vulnerable to lethal hyperthermia.