This compilation, prepared by the Historical Office, Office of the Secretary of Defense, presents information about the organization of the Department of Defense and its key officials since its establishment in 1947. Officials are listed in accordance with their status within the department, for the most part; the assistant secretaries and equivalent-level officials are listed according to organizational function. The table of contents and text include the names of some offices that have been superceded by new names or organizations. The text indicates all instances where this has occurred. In most entries the effective date of incumbency is the initial swearing-in date. Officials assume office after being sworn in, usually followed by a ceremonial swearing in, days or weeks later. Dates of Department of Defense directives confirming establishment of positions and prescribing functions usually follow appointments by months and sometimes years. Offices are often without a permanent appointee for periods ranging from days to years. During many of these periods there are acting officials in charge. In other instances there is no indication of an acting official and a time gap is evident. For the most part, acting officials have been de facto rather than formally designated.