The climate in the Arctic is changing faster than anywhere else on Earth. Poorly understood feedback processes relating to Arctic clouds and aerosol-cloud interactions contribute to a poor understanding of the present changes in the Arctic climate system, and also to a large spread in projections of future climate in the Arctic. The problem is exacerbated by the paucity of research-quality observations in the central Arctic. Improved formulation in climate models requires such observations, which can only come from in-situ observations in this region of logistical difficulty to reach and with its demanding environmental conditions. The Arctic Summer Cloud-Ocean Study (ASCOS) was the most extensive central Arctic Ocean expedition with an atmospheric focus during the International Polar Year (IPY). ASCOS focused on the study of the formation and life cycle of low-level Arctic clouds. ASCOS departed Longyearbyen, on Svalbard on 2 August and returned on 9 September, 2008. In transit into and out of the pack ice, four short research stations were undertaken in the Fram Strait, two in open water and two in the marginal ice zone. After traversing the pack-ice northward an ice camp was deployed on 12 August at N87˚21’ W01˚29’ which was in operation through 1 September, drifting with the ice. During this time extensive measurements were taken of atmospheric gas and particulate phase chemistry and physics, mesoscale and boundary-layer meteorology, marine biology and chemistry, and upper ocean physics. ASCOS provides a unique interdisciplinary data set for development and testing of new hypotheses on cloud processes, climate models and satellite retrieval algorithms.