This manual, Army Techniques Publication ATP 3-21.90 MCTP 3-01D Tactical Employment of Mortars October 2019, is a multiservice publication that provides Army and United States Marine Corps (USMC) a doctrinal reference for the employment of mortar squads, sections, and platoons. It contains guidance on tactics and techniques that mortar elements use to execute their part of combat operations described in battalion-, squadron-, troop-, and company-level manuals. This publication contains guidance on how a mortar element's fires and displacement are best planned and employed to sustain a commander's intent for fire support.The target audience for ATP 3-21.90/MCTP 3-01D includes mortar squad, section, and platoon leaders, company and battalion commanders, battalion staff officers, and all others responsible for controlling and coordinating fire support. Training developers use this manual as a source document for combat critical tasks. Combat developers use this manual when refining and revising operational concepts for mortar organizations. This publication serves as the primary reference for both resident and nonresident mortar tactical employment instruction.ATP 3-21.90 / MCTP 3-01D applies to the Active Army, Army National Guard, Army Reserve of the UnitedStates, and USMC.Chapter 1 discusses the overview of mortar systems, fire direction center systems, and duty positions for a mortar platoon or section in the Armored, Infantry, and Stryker Brigade Combat Team. This chapter discusses the role of mortars with an introduction to the purpose. Chapter 2 discusses an overview of fires organization and fire support plan and coordination with emphasis on the top-down bottom-up refinement. Chapter 3 establishes techniques and procedures for mortar platoons or sections in support of platoon, company, and battalion operations.Chapter 4 provides an overview of mortar platoon and section operations in support of company, troop, and battalion operations.Chapter 5 provides baseline survivability techniques for mortar platoons and sections in all operating environments. Chapter 6 covers the sustainment and logistics of mortar elements in support of operations. Appendix A discusses mortar ammunition utilization and effects on targets to include high explosive, phosphorus smoke, and illumination cartridges.