Southern Ocean marine mammals range widely and play an important role in marine food webs. Four species of Antarctic pinniped and two species of cetacean are entirely dependent on sea ice for key aspects of their life history. Another two species of pinniped and four species of cetacean are regularly associated with the Antarctic sea ice zone, foraging and traversing at the ice edge especially during the austral summer. Biological characteristics, threats to and conservation status of each species are presented. Climate change seems to be the primary threat to Antarctic marine mammals although its effects are likely to vary among species temporally and spatially. Loss of sea ice may variously reduce or increase access to foraging and breeding habitat but consequences of a continuing retreat of sea ice remain speculative. We present a case study of southern elephant seals to illustrate impacts of environmental shifts on their diving behaviour.