The Earth's climate has constantly evolved and changed throughout its 4.8 billion‐year history. During most of the Earth's history, climate has been characterized as hyperthermal, punctuated by cool periods during the Neoproterozoic, late Ordovician, early Permian, late Jurassic, and from the early Eocene to the present. These broad changes in climate occurred in response to a number of internal and external‐forcing mechanisms, including changes in solar output, changes in Earth's orbital geometry, meteor impacts, orogenesis and tectonic processes, and the variable concentrations of greenhouse gases in the atmosphere. Paleoclimatology is the science that studies these changes in climate throughout the entire history of Earth. The relevancy of paleoclimatology has grown in recent decades in response to anthropogenic climate change and the need to understand natural climate variability.