An oil spill is the release of a liquid petroleum hydrocarbon into the environment, especially the marine ecosystem, due to human activity, and is a form of pollution. The term is usually given to marine oil spills, where oil is released into the ocean or coastal waters, but spills may also occur on land. Oil spills may be due to releases of crude oil from tankers, offshore platforms, drilling rigs and wells, as well as spills of refined petroleum products and their by-products, heavier fuels used by large ships such as bunker fuel, or the spill of any oily refuse or waste oil.

The Lac-Mégantic rail disaster occurred in the town of Lac-Mégantic, Quebec, Canada, on July 6, 2013, at approximately 1:14 a.m. EDT, when an unattended 73-car Montreal, Maine and Atlantic Railway (MMA) freight train carrying Bakken Formation crude oil rolled down a 1.2% grade from Nantes and derailed downtown, resulting in the explosion and fire of multiple tank cars. Forty-seven people were killed. More than thirty buildings in Lac-Mégantic's town centre were destroyed, and all but three of the thirty-nine remaining buildings had to be demolished due to petroleum contamination. Initial newspaper reports described a 1 km (0.6-mile) blast radius.