The polar ice sheets of Greenland and Antarctica are unique sedimentary environments which offer valuable information on the climate and atmospheric environment of the past. The prevailing low temperatures, lack of mixing of strata, and relatively high accumulation rates result in high quality records which may be resolved in great detail, frequently at the scale of seasons of the year. Deformation of ice at great depth reduces the resolution but extends the available time span to hundreds of thousands of years. Climatic and environmental information is obtained from ice cores penetrating the ice sheets and is derived from the composition of the ice itself, as well as from chemical impurities and entrapped air bubbles. Ice core signals can be related to climatic variables such as mean annual temperature, summer warmth, and past precipitation rates; past environmental effects that can be detected from ice core data include volcanic eruptions, changes in solar activity, and the impact of man's activities on recent snow composition.