The Eocene was the second geological epoch in the Palaeogene. It began 56 million years ago, and ended 33.9 million years ago. Before it was the Palaeocene and, after it, the Oligocene.

The Eocene, like the Palaeocene before it, had a climate much warmer than today. At the start of the Eocene the Palaeocene–Eocene Thermal Maximum was reached. This lasted for 100,000 years, and caused a large extinction event. At the end of the Eocene another extinction event occurred, this time probably caused by meteorite strikes in Siberia and Chesapeake Bay.

The land was heavily forested, with temperate forests into the polar regions, and the many herbivorous mammals were browsers, not grazers.

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