There are two main types of turbine engine lubrication systems: wet-sump and dry-sump. Wet-sump systems store and circulate oil within the engine housing, while dry-sump systems use external tanks and pumps to store, cool, and circulate oil. Dry-sump systems are now more common and can be categorized as either hot-tank or cold-tank depending on whether oil is cooled before or after returning to the external storage tank. Both systems pressurize oil to lubricate engine bearings and gears and use scavenger pumps to return drained oil for recirculation.