Long-distance migration enables many organisms to take advantage of lucrative breeding and feeding opportunities during summer at high latitudes and then to move to lower, more temperate latitudes for the remainder of the year. The latitudinal range of the Adélie penguin spans ~22o. Penguins from northern colonies may not migrate, but due to the high latitude of Ross Island colonies, these penguins almost certainly undertake the longest migrations for the species. Previous work has suggested that Adélies require both pack ice and some ambient light at all times of year. Over a 3-yr period, which included winters of both extensive and reduced sea ice, we investigated characteristics of migratory routes and wintering locations of Adélie Penguins from two colonies of very different size on Ross Island, Ross Sea, the southernmost colonies for any penguin. We acquired data from 3-16 Geolocation Sensors affixed to penguins each year at both Cape Royds and Cape Crozier in 2003-2005. Migrations averaged 12,760 km, with the longest being 17,600 km, and were in part facilitated by pack ice movement. Trip distances varied annually, but not by colony. Penguins rarely traveled north of the main sea ice pack, and used areas with high sea-ice concentration, ranging from 75-85%, about 500 km inward from the ice edge. They also used locations where there was some twilight (2-7 hr with sun < 6° below horizon). We report the present Adélie Penguin migration pattern and conjecture on how it likely has changed over the past ~12,000 years, as the West Antarctic Ice Sheet withdrew southward across the Ross Sea, a situation that no other Adélie penguin population has had to confront. As sea ice extent in the Ross Sea sector decreases in the near future, as predicted by climate models, we can expect further changes in the migration patterns of the Ross Sea penguins. (Ecology, 91 (7) (2010): 2056–2069)