This publication, Army Techniques Publication ATP 3-21.20 Infantry Battalion December 2017, provides doctrine for the Infantry battalion of the Infantry brigade combat team (IBCT). ATP 3-21.20 describes relationships, organizational roles and functions, capabilities and limitations, and responsibilities within the Infantry battalion. Techniques, nonprescriptive ways or methods used to perform missions, functions, or tasks (CJCSM 5120.01), are discussed in this publication and are intended to be used as a guide. The techniques are not prescriptive. ATP 3-21.20 publication supersedes FM 3-21.20 and FM 3-21.12. Readers must first understand the principles of the Army profession and the Army ethic as described in ADRP 1 to comprehend the doctrine contained in this publication. Readers must understand the principles of joint operations, the nature of unified land operations, and the links between the operational and tactical levels of war described in JP 3-0, ADP 3-0, and ADRP 3-0; FM 3-94, ATP 3-91, and FM 3-96. In addition, readers should understand the fundamentals of the operations process found in ADP 5-0 and ADRP 5-0 associated with offensive and defensive tasks contained in FM 3-90-1 and reconnaissance, security, and tactical enabling tasks contained in FM 3-90-2. The reader must comprehend how stability tasks described in ADP 3-07 and ADRP 3-07 carry over and affect offensive and defensive tasks and vice versa. Readers must understand how the operation process fundamentally relates to the Army's design methodology, military decision-making process, troop-leading procedures, and the principles of mission command as described in ADP 6-0, ADRP 6-0, FM 6-0, and ATP 6-0.5. The principal audience for ATP 3-21.20 is the commanders, staff, officers, and noncommissioned officers within the Infantry battalion. The audience also includes the United States Army Training and Doctrine Command institutions and components, and the United States Army Special Operations Command. This publication serves as an authoritative reference for personnel developing doctrine, materiel and force structure, institutional and unit training, and standard operating procedures for the Infantry battalion.