Cores drilled through the polar ice sheets provide information about past climate and environmental conditions on timescales from decades to hundreds of millennia, and direct records of changes in the composition of the atmosphere. As such, they are cornerstones of global change research. In the past 15 years, several major projects have increased our understanding of past climate change on a variety of timescales. These include the Greenland Ice Sheet Project 2 (GISP2), the Greenland Ice Core Project (GRIP), and North GRIP deep ice cores in Greenland. They also include the Taylor Dome, Siple Dome, Law Dome, Vostok, and European Programme for Ice Coring in Antarctica (EPICA) Dome C ice cores in Antarctica, the latter of which has pushed the record for the oldest ice core back to 720,000 years.