1-17. Maintenance is all actions taken to retain materiel in a serviceable condition or to restore it to serviceability. The Army utilizes a tiered, two-level maintenance system composed of field and sustainment maintenance. Command teams, maintenance personnel and planners must have a complete understanding of two-level maintenance fundamentals in order to properly plan and execute their mission. Maintenance is necessary for endurance and performed at the tactical through strategic levels of war. See ATP 4-33 for more information.