The term ice ages usually refers to the recurring periods of glaciation during the Pleistocene and Holocene epochs of the Quaternary period. During these times, glaciers advanced from the north across North America and Eurasia. The Quaternary period has been one of the coldest in all of Earth history, but glaciations also occurred many times in the past. For example, glaciations occurred in Gondwana 355 million to 280 million years ago and in Antarctica beginning 33 million years ago. Perhaps the most extensive ice ages occurred during the Precambrian time. This entry focuses upon the recurring Northern Hemisphere ice ages of the Quaternary period.

An interglacial period followed each ice age, in which the glaciers melted and retreated. Starting about 950,000 years ago, each ice age plus interglacial period lasted about 100,000 years. During the Pleistocene epoch, about 20 ice ages occurred, with the Holocene epoch consisting of the time since the peak of the most recent glaciation. In many cases, a more recent glaciation has obliterated the geological evidences of an earlier one. Several of the more extensive glaciations can be directly studied from geological deposits. The recent glaciations have been named for the location of their maximum extent, the same glaciations receiving different names in Europe and in North America.

Major credit for the discovery of the Ice Ages goes to Louis Agassiz but Agassiz had predecessors. Jean de Charpentier, for example, observed that boulders that were out of place had to be moved by some massive force that did not involve the movement of the crust itself; he believed such a force could only have been caused by glaciers. The botanist Karl Schimper invented the concept of Eiszeit, which translates to Ice Age. Most scientists, such as Alexander von Humboldt, Roderick Murchison, and even Charles Lyell, the father of uniformitarianism, opposed the idea when it was first proposed. Before this theory could be credible, a believable cause needed to be proposed and evidence for it presented. This was the same problem that evolutionary theory had, at about the same time. Despite the mystery surrounding what may have caused the ice ages, Agassiz assembled the evidence that at least one ice age had occurred. His evidence, including everything from boulders out of place to the rubble and bones found in caves, turned the tide of opinion. Interestingly, Agassiz believed in catastrophism, that Earth history consisted of a series of catastrophes. Earlier catastrophists had maintained that the Flood of Noah was the most recent of these catastrophes; Agassiz changed his mind, and those of other catastrophists, by concluding that this "flood" had been ice, not liquid water. . .

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