The Marine Corps is responsible for amphibious warfare and expeditionary warfare operations, having a very close relationship with its coequal sister service, the United States Navy. The U.S. Marine Corps is composed of the Regular Marine Corps and the United States Marine Corps Reserve. The central unit of the Marine Corps is a Marine Air-Ground Task Force, which consist of a command element, ground combat element, aviation combat element, and logistics combat element. The Marine Corps is divided in the Fleet Marine Force and the Supporting Establishment.