Spending for operation and maintenance (O & M) supports the military services' day-to-day activities, such as the training of military units, maintenance of equipment, recruitment of service members, operations of military bases, and provision of administrative services. In 2010, appropriations for O & M (excluding funds for the Defense Health Program) totaled $157 billion and constituted some 29 percent of the Department of Defense's "base" budget. DoD typically cites the readiness of military units to perform their missions in wartime as the primary justification for its O & M budget requests to the Congress. DoD, however, has not been able to clearly identify the relationship between the department's O & M spending and the readiness of military units. Nor has the Congressional Budget Office's analysis, which used historical data to attempt to establish statistical relationships between O & M spending and readiness for selected units, yielded a well-defined linkage. (CBO's analysis focused only on unit readiness because of the role it plays in DoD's assessments of the services' need for O & M funding.) Those efforts were not fruitful, largely because the information needed to determine that linkage, effective measures of readiness and detailed data on spending, is not readily available or may not, in fact, exist. The military's current measures of readiness are not readily applicable to such analyses, and there are some concerns about the quality of its assessments of readiness. Yet even if readiness were well measured, determining the relationship between readiness and O & M spending presents challenges. Some activities supported by O & M spending may be more directly related to a unit's current readiness than other such activities are; in addition, some spending from other types of appropriations may affect readiness. Also, spending intended to support units' readiness activities must be distinguished from spending for overseas contingency operations. If DoD is to determine how O & M spending affects units' readiness, it may have to conduct controlled experiments in which it methodically varies readiness-related spending for otherwise similar units.