Sean ice plays a key role for climate and is important as habitat and for human activities and economies. Observations show and models indicate that climate and sea-ice regimes are changing. Sea-ice extent in the Arctic decreased substantially during the last 30 years; Antarctic sea ice is decreasing in some areas, but overall it has shown a slight increase during this period. Climate models project further decreases in sea-ice extent in the Arctic during this century and comparable decreases in Antarctic sea-ice extent. There are uncertainties attached to the rate at which these changes will occur, and there is a risk of tipping points being crossed and abrupt reductions in sea ice occurring. To reduce these uncertainties, more large-scale continuous observations are needed, especially of ice and snow thickness.