Gases trapped in polar ice sheets provide a means of obtaining samples of Earth's atmosphere from times in the past. We describe geochemical processes affecting gases trapped in ice and discuss how these mechanisms affect the composition of gas extracted from polar ice cores. We review the methods for recovering samples of paleo-atmospheres from ice cores and discuss the suitability of each method in different applications. We conclude that extracting gas by mechanical disintegration (e.g., crushing, grinding, shaving, or milling) is most easily performed on relatively young ice in which the content of the bubbles is still representative of the original atmosphere. For ice recovered from greater depth, where some or all of the gases have been driven into the ice matrix itself, more care must be taken in interpreting the analysis of the extracted gases. ¿ 1997 American Geophysical Union