This publication 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. Tables have been updated as of the report's issuance date to reflect leadership changes. 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. In the earliest Congresses, those House Members who took some role in leading their party were often designated by the President as his spokesperson in the chamber. By the early 1800s, an informal system developed when the Speaker began naming his lieutenant to chair one of the most influential House committees. Eventually, other members wielded significant influence via other committee posts (e.g., the post-1880 Committee on Rules). By the end of the 19th century, the formal position of floor leaders had been established in the House. The Senate was slower than the House to develop formal party leadership positions, and there are similar problems in identifying individual early leaders. For instance, 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 chairs 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. Rather, through the turn of the 20th century, individuals who led the Senate achieved their position through recognized personal attributes, including persuasion and oratorical skills, rather than election or appointment to formal leadership posts. The formal positions for Senate party floor leaders eventually arose from the position of conference chair. Owing to the aforementioned problems in identifying informal party leaders in earlier Congresses, the tables in this report identify each leadership position beginning with the year in which each is generally regarded to have been formally established. The publication excludes some leadership posts in order to render the amount of data manageable. A bibliography cites useful references, especially in regard to sources for historical data, and an appendix explains the abbreviations used to denote political parties.