The histories of two national libraries, the British Museum Library and the Library of Congress, are examined with respect to the development of each of three functions: (1) the acquisition and maintenance of a comprehensive collection of the country's publications, usually by copyright deposit; (2) the maintenance of basic research collections in the major fields of inquiry; and (3) the provision of leadership and assistance to the rest of the country's libraries in bibliographic activities. It is suggested that, despite dissimilar origins and varying rates of development, the two libraries came eventually to perform the same functions, and thus that the development of national libraries is guided less by convention or tradition than by the needs of the societies in which they arise. (Author/JD)