The Department of Defense (DOD) has long relied on contractors to provide the U.S. military with a wide range of goods and services, including weapons, food, uniforms, and operational support. Without contractor support, the United States would be currently unable to arm and field an effective fighting force. Understanding costs and trends associated with contractor support could provide Congress more information upon which to make budget decisions and weigh the relative costs and benefits of different military operations -- including contingency operations and maintaining bases around the world. Obligations occur when agencies enter into contracts, employ personnel, or otherwise commit to spending money. The federal government tracks money obligated on federal contracts through a database called the Federal Procurement Data System-Next Generation (FPDS). There is no public database that tracks DOD contract outlays (money spent) as comprehensively as obligations. This book examines how much money DOD obligates on contracts; what DOD is buying; and where that money is being spent. This book also examines the extent to which these data are sufficiently reliable to use as a factor when developing policy or analyzing government operations. In addition, this book provides background information and identifies issues for Congress on the use of contractors to support military operations. DODs extensive use of contractors poses several potential policy and oversight issues for Congress and has been the focus of numerous hearings. Congress' decisions on these issues could substantially affect the extent to which DOD relies on contractors in and is capable of planning for and overseeing contractors in future operations.