A glacial period (or alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate within an ice age. The last glacial period ended about 15,000 years ago; The Holocene epoch is the current interglacial.

Within the Quaternary glaciation (2.58 Ma to present), there have been a number of glacials and interglacials.

In the British Isles the Pleistocene extent of the Quaternary ice age, has been subdivided into the following stages and superstages.

The Cromerian consists of multiple glacial and interglacial periods. Other stages, the Anglian and Hoxnian, consist either of single glacial or interglacial periods. This subdivision is valid for the British Isles and hence has a local significance. Other areas have different subdivisions that partly correlate with the British subdivision.

The last glacial period was the most recent glacial period within the current ice age, occurring in the Pleistocene epoch, which began about 70,000 and ended about 12,500 years ago. The glaciations that occurred during this glacial period covered many areas of the Northern Hemisphere, and have different names, depending on their geographic distributions: Wisconsin (in North America), Devensian (in Great Britain), Midlandian (in Ireland), Würm (in the Alps), Weichsel (in northern central Europe) and Llanquihue in Chile. The glacial advance reached its maximum extent about 18,000 BP. In Europe, the ice sheet reached northern Germany.