Even a small amount of oil in the sea can have severe effects on marine life depending on the location and timing of its release. Changes in regulations and industry practices following the Exxon Valdez spill in 1989 have helped to dramatically reduce oil inputs from petroleum transportation and extraction, but most oil that enters the sea does not come from oil exploration, extraction, or spills. In fact, nearly 85 percent of the 29 million gallons of petroleum that enter North American ocean waters each year as a result of human activities comes from land-based runoff, polluted rivers, airplanes, and small watercraft. This report, which is the third in a series, presents the most up-to-date estimates of major oil sources and makes recommendations to help policy-makers prioritize next steps for prevention and response.