Book excerpt: Briefly describes current responsibilities and selection mechanisms for 15 House and Senate party leadership posts and provides tables with historical data, including service dates, party affiliation, and other information for each. Although party divisions appeared almost from the First Congress, the formally structured party leadership organizations now taken for granted are a relatively modern development. Constitutionally-specified leaders, namely the Speaker of the House and the President pro tempore of the Senate, can be identified since the first Congress. Other leadership posts, however, were not formally recognized until about the middle of the 19th century, and some are 20th century creations. Bibliography. Tables. A print on demand report.