Glaciation, often called an ice age, is a geological phenomenon in which massive ice sheets form in the Arctic and Antarctic and advance toward the equator. Glaciations are characterized by cool, wet climates and thick ice sheets extending from each pole. Mountain or alpine glaciers in otherwise unglaciated areas expand and extend to lower elevations even in the lowest of latitudes. Sea levels drop due to the presence of large volumes of water above sea level in the icecaps. There is evidence that ocean circulation patterns are disrupted by glaciations. Since the earth has significant continental glaciation in the Arctic and Antarctic, we currently are in a glacial minimum of a glaciation.

In general, the Earth seems to have been ice free even in high latitudes except during relatively rare glacial maximums such as the one from which we emerged from 10 to 15 thousand years ago.

The causes of glaciations have been much debated ever since the phenomenon was clearly identified in the 17th century. Modern theories tend to revolve around periodic oscillations in the Earth's orbit; hypothecised periodic changes in solar output; and/or the effects of continental masses drifting into polar regions where Antarctica currently resides.

Known periods of glaciation include the Huronian (2400Ma-2100Ma), Sturtian-Varangian (950Ma-570Ma), Andean-Saharan (450Ma-420Ma), Karoo (360Ma-260Ma), Cenozoic (30Ma-Present). These can be further divided by location and time (eg: the names Riss (180,000 - 130,000 years bp) and Wurm (70,000-10,000 years bp) refer specifically to glaciation at the headwaters of the Donau / Rhine rivers.) Not every year in each interval was a time of complete or even partial glaciation. The best studied glaciation, that of the recent past, appears to have taken place in at least four separate ice incursions and retreats. Unfortunately the scouring action of each glaciation tends to remove most of the evidence of prior icesheets almost completely, except in regions where the later sheet doesn't achieve full coverage. It is probable that glacial periods other than those above have been overlooked because of the paucity of exposed rocks from high latitudes from older time periods. The Varanger glaciation was especially severe and may have extended to the Equator. This has led to a recent Snowball Earth hypothesis that the Earth froze over completely in the late Proterozoic, then thawed very rapidly as trapped water and carbon dioxide were returned to Earth's atmosphere. An alternative hypothesis, sometimes called Slushball Earth, maintains that the Equator at least was ice-free.

An ice age is sometimes used to refer to a single glaciation, or to an entire period of repeated glaciations such as the recent 30 million years of the Cenozoic period, especially the Pleistocene glaciations.