This publication applies to the Active Army, the Army National Guard (ARNG)/Army National Guard of the United States (ARNGUS), and the United States Army Reserve (USAR) unless otherwise stated.The doctrine of gap-crossing operations focuses on the support provided by engineer and other capabilities to the combined arms team that enhances mobility of the force by projecting elements across an obstacle, wet or dry, in support of assured mobility. It is also applicable to joint, interagency, or multinational forces and is specifically written as a dual manual between the United States (U.S.) Army and the U.S. Marine Corps (USMC). Although other branches contribute to gap-crossing operations and are included in the discussions, this manual focuses on the engineer contribution to gap-crossing operations, while acknowledging the significant role of other branches and capabilities. This manual follows the principles and tenets found in Field Manual (FM) 3-90, FM 3-34, and FM 3-34.2 that continues the discussion of mobility and gap-crossing operations. It recognizes the contribution of the entire combined arms team to gap-crossing operations and the multi-Service capabilities that exist to support gap-crossing operations at both the tactical and operational levels. Finally, it addresses the specifics associated with gap crossing in support of combat maneuver and line of communications (LOC) gap crossing, integrating the considerations created by the significant changes to doctrine and force structure that have occurred since FM 90-13 was published in 1998.