Ice cores are an excellent way of finding out how the atmosphere has changed because climate signals and many forcing factors for climate are present in the same core. Their main disadvantage is that they are only available in the polar regions and at a few unusual sites elsewhere. Cores range from short ones designed to understand seasonal deposition up to the 6.6 km deep/400 kyr record from Vostok, Antarctica. Signals exist in the water isotopes, in the soluble and insoluble impurities, and in the trace gases trapped in air bubbles. On short timescales, ice cores are the only means by which we know how greenhouse gas concentrations have altered in the last few centuries. On long timescales, ice cores have shown how temperature and greenhouse gases track each other through climatic cycles. They have also been responsible for revealing the very rapid switches in climate (Dansgaard-Oeschger events) that occurred during the last glacial period.