Oil spills can occur on land or in rivers and lakes, but the most serious spills tend to involve tankers that travel the open seas. Spills represent a serious environmental risk as they threaten a wide range of marine species that can die when they or the food they need become coated with oil. The largest oil spill in US history occurred when the tanker Exxon Valdez ran aground in Prince William Sound in Alaska in 1989, dumping about 260,000 barrels of oil, which eventually affected about 1,100 miles of Alaskan coastline.

In response to the spill, Congress passed the Oil Pollution Act of 1990, which enabled the federal government to issue new rules to prevent spills (such as ensuring that tanker captains and crew are fully qualified and not operating under the influence of drugs or alcohol, and limiting where tankers can operate) and which established the liability and financial responsibilities of any entity involved in an oil spill. Advances continue to be made in the technology to clean up oil spills, including the use of bacteria that can break down the oil into less harmful substances.