The Palaeocene-Eocene Thermal Maximum (PETM), a geologically brief episode of global warming associated with the Palaeocene-Eocene boundary, has been studied extensively since its discovery in 1991. The PETM is characterized by a globally quasi-uniform 5-8 °C warming and large changes in ocean chemistry and biotic response. The warming is associated with a negative carbon isotope excursion (CIE), reflecting geologically rapid input of large amounts of isotopically light CO2 and/or CH4 into the exogenic (ocean-atmosphere) carbon pool. The biotic response on land and in the oceans was heterogeneous in nature and severity, including radiations, extinctions and migrations. Recently, several events that appear similar to the PETM in nature, but of smaller magnitude, were identified to have occurred in the late Palaeocene through early Eocene, with their timing possibly modulated by orbital forcing. Although debate continues on the carbon source, the mechanisms that caused the input, the mechanisms of carbon sequestration, and the duration and pacing of the event, the research carried out over the last 15 years has provided new constraints and spawned new research directions that will lead to improved understanding of PETM carbon cycle and climate change.