This book presents tables that provide historical data, including service dates, party affiliation, and other information, for 15 House and Senate party leadership posts. Although party divisions appeared almost from the First Congress, the formally structures party leadership organisations 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 officially recognised until about the middle of the 19th century, and some are 20th century creations. The Senate was slower than the House to develop a separate, identifiable party leadership. Records of party conferences in the 19th century Senate are not available. Memoirs and other secondary sources reveal the identities of party conference or caucus chairmen for some, but not all, Congresses after about 1850; but these posts carried very little authority. It was not uncommon for Senators to publicly declare that within the Senate parties, there was no single leader. achieved their position through recognised personal attributes, including persuasion and oratorical skills, rather than election or appointment to official leadership posts. The development of Senate party floor leaders was, like in the House, one of slow evolution, linked for the most part to the post of conference chairman. Not until 1945 did Senate Republicans specify that the conference chairmanship and floor leader posts must be held by separate Senators. Among Senate Democrats, the floor leader is also chairman of the conference. A special section presents biographical sketches of each of the leaders.