Long ice cores from Greenland yield records of annually resolved climate change for the past ten to twenty thousand years, and decadal resolution for one hundred thousand years or more. These cores are ideally suited to determine the rapidity with which major climate changes occur. The termination of the Younger Dryas, which marks the end of the last glacial period, appears to have occurred in less than a human lifetime in terms of oxygen isotopic evidence (a proxy for temperature), in less than a generation (20 years) for dust content and deuterium excess (proxies for winds and sea-surface conditions), and in only a few years for the accumulation rate of snow. Similarly rapid changes have been observed for stadial-interstadial climate shifts (Dansgaard-Oeschger cycles) which punctuate the climate of the last glacial period. These changes appear to be too rapid to be attributed to external orbital forcings, and may result from internal instabilities in the Earth's atmosphere-ocean system or periodic massive iceberg discharges associated with ice sheet instability (Heinrich events). In contrast, the Holocene climate of the Arctic appears to have been relatively stable. However, the potential for unstable interglacials, with very rapid, shortlived climatic deteriorations, has been raised by results from the lower part of the GRIP ice core. These results have not been confirmed by other ice cores, notably the nearby GISP2 core. Evidence from other records of climate during the Eemian interglacial have yielded mixed results, and the potential for rapid climate change during interglacial periods remains one of the most intriguing gaps in our understanding of the nature of major Quaternary climate change.