Critical habitat for marine mammals is defined in terms of the functioning ecological units required for successful breeding and foraging. This definition is used to consider the potential effects of environmental change in the 21st century on abundance and distribution of marine mammals. Critical habitat for breeding can be identified relatively easily for pinnipeds and some coastal and freshwater cetaceans. Critical habitat for foraging is more difficult to define, particularly for pelagic species. However, telemetry-based studies have indicated that relatively localized areas may be particularly important for some species. Habitat degradation, as a result of reduction in prey density and increased risks of mortality due to human activity, is likely to be a problem for most species. Ice-breeding seals, particularly those that are endemic to inland seas and large lakes, are most likely to be affected by climate change. Climate change will also affect distribution and availability of prey in the short and long term. Although highly mobile species, such as marine mammals, can respond more rapidly to effects of climate change than their terrestrial counterparts, central-place foragers, such as many otariid seals, may still be seriously affected.