The formation of precipitation is a complex process by which suspended atmospheric moisture in the form of very small cloud droplets ultimately transitions into the larger frozen or nonfrozen precipitation observed at the surface. This entry describes four aspects of precipitation formation. First, the presence of a lifting mechanism is necessary to lift the air vertically. Second, condensation or ice nuclei are essential for the formation of precipitation in the atmosphere. Third, there are two fundamental processes by which precipitation forms; both of these processes are described and referenced by their typical geographic locations of occurrence. Finally, the unique formation of particular precipitation types is discussed.