This manual establishes the techniques used by individuals, teams, and units of United States (U.S.) Army and U.S. Marine Corps (USMC) Civil Affairs (CA) forces, as well as planners of civil-military operations (CMO) at the strategic, operational, and tactical levels of war. The techniques prescribed in this manual are used when engaging other government agencies (OGAs), indigenous populations and institutions (IPI), intergovernmental organizations (IGOs), nongovernmental organizations (NGOs), and other military and nonmilitary entities in support of conventional and special operations missions. This manual elaborates on doctrine contained in Field Manual (FM) 3-57, Civil Affairs Operations. As with all doctrinal manuals, Army Techniques Publication (ATP) 3-57.20/Marine Corp Reference Publication (MCRP) 3-33.1C, Multi-Service Techniques for Civil Affairs Support to Foreign Humanitarian Assistance, is authoritative, but not directive. It serves as a guide and does not preclude CA personnel or units from developing their own standing operating procedures. The techniques presented in this manual should not limit CA forces from using their civilian-acquired skills, training, and experience to meet the challenges they face while conducting Civil Affairs operations (CAO) and providing support to CMO. Appendixes A, B, and C provide the users of this manual with additional information. The principal audience for ATP 3-57.20/MCRP 3-33.1C is Army and USMC CA forces, officers, and noncommissioned officers who support joint and Army forces or serve on the staff that support commanders and operations at all levels of war. It is also an applicable reference to the civilian leadership of the United States (U.S.) interagency. This publication applies to the Active Army, Army National Guard/Army National Guard of the United States, United States Army Reserve, USMC, and Marine Corps Reserve unless otherwise stated.