When oil is spilled, whether on water or land, a number of transformation processes occur, which are referred to as the “behavior” of the oil. Two types of transformation processes are discussed in this chapter. The first is weathering, a series of processes whereby the physical and chemical properties of the oil change after the spill. The second is a group of processes related to the movement of oil in the environment. Spill modeling is also included in the section on oil movement. Weathering and movement processes can overlap, with weathering strongly influencing how oil is moved in the environment. These processes depend very much on the type of oil spilled and the weather conditions during and after the spill.