The Series Commander is a United States Marine Corps officer assigned to a Recruit Training Company who is responsible for the conduct and instruction of Marine recruits and Marine Corps Drill Instructors within one of the two series, lead or follow. Series Commanders are typically of the rank of First Lieutenant (O-2) or captain (O-3), and they are assisted in their duties by a Series Chief Drill Instructor (MCRD San Diego) or a Series Gunnery Sergeant (MCRD Parris Island).