As a legislative organization, the United States Congress has come increasingly to be characterized by the size and expertise of its committee staffs. Primarily based upon extensive interviews with congressional staff personnel, this paper deals with four major questions: In recent decades, how have committee staffs developed? Who are the major users of committee staff personnel? What are the main capabilities of committee staffs? And, what are the principal biases or constraints in the congressional context which shape the behavior of staff personnel? Congressional committee staffs have grown considerably in size in the postwar years, their work is mainly oriented to the committee members themselves, the committee staffs have come to be very influential, and the boundaries of their activities are well-defined by the constraints operating in the congressional institution.