A brief review of the breeding biology of the Weddell seal, Leptonychotes weddelli is presented, with a discussion of factors which regulate the sizes of the populations. In early spring breeding colonies form on sea-ice as pregnant females haul out in areas of perennial cracking. The period between hauling out and birth of the pup, a time of fasting for the female, apparently increases in length with latitude and is generally later at higher latitudes. Development of the Single pup i s rapid; i t doubles i t s weight in 10 days and reaches 114 kg weight before weaning at 6 to 7 weeks of age. Twinning is rare. The lanugo of the pup provides insulation in the absence of blubber and allows the pup to survive at temperatures well below freezing. Mortality of pups, highest in the first week of life, is largely determined by climatic and physical factors. The pup's diving and swimming proficiency develops fairly rapidly but appears to necessitate a transitional crustacean diet following weaning. Weaning at high latitudes is thought to be timed to coincide with the summer phytoplankton bloom. The parent-offspring bond, initially strong, is broken about 50 days after birth and pups are forced, through strong competition with adults for food, to retreat towards the pack-ice. In breeding colonies seals are spaced wider than at other times on the sea-ice because of territorial behaviour. Both sexes defend aquatic territories while breeding but there is little fighting. Populations of Leptonychotes exist as discrete units. Food is probably the ultimate factor limiting population size but other important factors are the availability of breathing holes and the ability to maintain these. Intra-specific and intra-sexual strife are also significant. Ice stability can have large effects on pupping success. Predation may be important in some areas but is poorly documented.