The air trapped in freshly formed ice gives information concerning the ice formation processes as well as concerning several environmental parameters at the time of ice formation. Air amount, air composition, and the size and form of bubbles may change with time. Possible processes responsible for such changes are discussed. In very cold ice air content and air composition remain almost unchanged. Samples of ancient atmospheric air are therefore very well preserved in cold ice. In temperate ice changes of the air amount and air composition depend on the intergranular water flow through the glacier. This waterflow can be estimated by measuring air amount and air composition in ice samples.