Earth's present climate system is unusual from a deep-time perspective because large ice sheets exist at both poles. Over the past 30 million years, the planet has shifted from a warmer greenhouse world into an icehouse climate marked by the growth of Antarctic ice, the later expansion of Northern Hemisphere ice sheets, and the repeated glacial-interglacial cycles of the Quaternary Period. These changes were driven by a complex interaction of plate tectonics, atmospheric carbon dioxide, ocean circulation, orbital variations, ice-sheet dynamics, and feedbacks involving snow, ice, vegetation, and dust. This presentation will trace the history of Earth's ice ages from the onset of major Antarctic glaciation around 34 million years ago to the great Pleistocene ice sheets that repeatedly covered large parts of North America and Eurasia. We will examine how scientists reconstruct past ice ages using marine sediments, oxygen isotopes, glacial landforms, ice cores, fossil evidence, and sea-level records. Particular attention will be given to why ice sheets grow and collapse, why glacial cycles changed in rhythm through time, and how relatively small changes in forcing can be amplified by powerful Earth-system feedbacks. The final part of the presentation will connect this long history to the present and future cryosphere. Modern warming is rapidly reducing mountain glaciers, Arctic sea ice, parts of the Greenland and Antarctic ice sheets, seasonal snow cover, and permafrost. These changes affect sea level, freshwater resources, ecosystems, coastal communities, ocean circulation, and climate feedbacks. By placing today's cryospheric change in the context of Earth's ice age history, we can better understand both the natural behavior of ice sheets and the exceptional pace of change now underway.