The International Polar Year 2007–2008 (IPY) provided both the incentive and the opportunity for a burst of research activity in the sea-ice zone of both the Arctic and Antarctic that would not have otherwise occurred. Of about 130 endorsed IPY science projects that were implemented between March 2007 and March 2009, more than 30% involved sea ice. This presentation provides an overview of IPY sea-ice activities, the international collaborations forged to undertake them, interrelationships between projects and lessons for the future. IPY sea-ice projects had a strong interdisciplinary emphasis and involved widespread sharing of logistical facilities, research capabilities and data. They included major sea-ice process studies in both the Arctic and Antarctic; studies of the role of sea ice in polar ecosystems, in the global climate system and in biogeochemical cycles; palaeo-environmental studies of past sea-ice conditions; investigations of interactions between sea ice, ice shelves and ice sheets; initiation of new observational systems and compilation of new databases; and examination of the influence of sea ice on Arctic societies. The full scientific legacy of these and other IPY projects will evolve in years to come, but already significant advances in scientific knowledge and understanding are emerging, including many that will be presented at this meeting. IPY took place during a time when Earth has been changing rapidly, particularly in the Arctic, and it highlighted the global importance of sea-ice processes and the urgency to understand and track the rapid changes occurring at high latitudes. It also set directions for priority polar research over the next decade and beyond, and initiated enhanced ocean and ice observations in both polar regions, which need to be developed into sustained polar observational systems.