As the major energy source worldwide, petroleum products have been increasingly produced and consumed, and the environmental impact involved also rises. Despite significant progress in reducing spillagethrough a variety of technological and regulatory prevention measuresand better industry practices, the risk of significant oil spills remains. On a daily basis, hundreds to thousands of spills are likely to occur worldwide including in the oceans. Marine oil spills involve accidental release of oil ranging from various types of crude oil to a large array of refined products, from heavy persistent fuels to light, less persistent, but very toxic fuels. The chemical composition and bulk properties of the spilled oil, and the associated weathering reactions, govern their fate, behavior, and impact in the marine environments. Oil spills are of great concern due to the enormous economic loss and the long-term, significant harm to marine ecosystems, local economy, and coastal society and community. During the long period of oil and gas exploitation, the adverse impacts of marine oil spill have been documented in various aspects. This chapter summarizes facts and effects of oil spills in oceans, and based on this, the response strategies and policies can be improved.