This United States Marine Corps manual, Marine Corps Reference Publication MCRP 12-10A.1 (3-35.1A) Small Unit Leader's Guide to Mountain Warfare Operations Change 1 April 2018, is a reference for all Marine leaders (team through company) to use when conducting operations in mountainous terrain, snow, cold weather, and/or high altitude. Topics include the environment, specialized clothing and equipment, weapons considerations, patrolling considerations, route selection and navigation, basic avalanche hazard assessment and mitigation, crossing streams/ice, snowshoeing, winter camouflage, logistics, fire support, helicopter operations, and casualty evacuation. It stresses only the skills necessary for Marines to function in combat. Because of the rapid turnover in personnel, operational tempo, and the multitude of training commitments, the Marine Corps Mountain Warfare Training Center cannot train all Marines or units. In combat, Marines may need to learn through doctrinal references that are augmented with instruction from their unit's qualified summer and winter mountain leaders. This MCRP gives them that capability. This publication supersedes MCRP 3-35.1A, Small Unit Leader's Guide to Cold Weather Operations, dated 16 November 1990.