The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present. The relatively short period is characterized by a series of glaciations, the appearance and expansion of anatomically modern humans, and their continuing impact on the natural world. The Quaternary includes two geologic epochs: the Pleistocene and Holocene. A proposed but as of yet informal third epoch, the Anthropocene, has also gained credence as the time in which humans began to profoundly affect and change the global environment, although its start date is still disputed.

The term Quaternary ("fourth") was proposed by Giovanni Arduino in 1759 for alluvial deposits in the Po river valley in northern Italy. It was introduced by Jules Desnoyers in 1829 for sediments of France's Seine Basin that seemed clearly to be younger than Tertiary Period rocks. The Quaternary Period follows the Tertiary Period and extends to the present. The Quaternary covers the time span of glaciations classified as the Pleistocene, and includes the present interglacial period, the Holocene. This places the start of the Quaternary at the onset of Northern Hemisphere glaciation approximately 2.6 million years ago. Prior to 2009, the Pleistocene was defined to be 1.805 million years ago to present, such that the present definition of the Pleistocene includes a portion of what was prior to 2009 defined as part of the Pliocene.