All spills are different, varying in type and amount of oil spilled, species exposed, and geographic and atmospheric conditions. It is important to understand as much as possible about both the natural history and characteristics of various species and the specific effects oil has on wildlife. Doing so improves the ability to extrapolate from one spill to another and improves prediction of types and severity of effects to wildlife. This chapter presents an overview of the biological effects of oil on marine mammals and sea turtles.