n fiscal year 2015, the Department of Defense (DoD) spent a total of roughly $390 billion on operation and support (O&S) of military units. The O&S budget covers the costs associated with the day-to-day running of units. Those costs include pay and benefits for military personnel, compensation for most civilian employees, health care costs for military and civilian personnel, and the daily expenses of operating a unit, such as equipment maintenance, training, support contractors, and so on. The O&S budget makes up about two-thirds of DoD's total "base" budget, which is the defense budget excluding additional funds provided specifically for wartime operations. (The rest of DoD's base budget is spent on acquiring weapon systems and constructing buildings and other infrastructure.) The size and complexity of the U.S. armed forces can make it difficult to determine how the O&S budget is distributed among units. In this report, the Congressional Budget Office analyzes the structure and cost of the military from the perspective of major combat units, such as Army brigades, Navy aircraft carrier strike groups, Marine Corps task forces, and Air Force squadrons. CBO allocates most of the O&S budget and DoD's total number of military personnel among major combat units- and their associated support units and overhead activities- to provide a clearer picture of the size and cost of the major elements of the military's force structure. Such information can help policymakers evaluate proposals to change the structure or budget of the armed forces. CBO's analysis indicates that major combat units by themselves account for roughly one-quarter of DoD's operation and support costs and contain about one-third of DoD's military personnel. Most of the rest of DoD's O&S costs and military personnel are associated either with units that support major combat units (which CBO considers part of the cost of maintaining fully supported major combat units) or with overhead activities necessary for manning, equipping, and training combat and support units. In addition, the total operating costs associated with a major combat unit include a share of the costs of "defensewide" activities, such as the Defense Health Program, that provide various forms of administrative support to DoD as a whole.