Seniority is based either on the length of continuous service in the chamber, or, for committee purposes, continuous service on a specific committee. Seniority is still the single best predictor of which Members will obtain leadership positions in Congress, but more so in the Senate than in the House. Experience and the political skills that come with time still count and engender trust and deference among colleagues. The problem with seniority usually occurs with the length of tenure in the top positions. Once achieved, removal of senior Members past their prime becomes the problem. To address this concern, House and Senate Republicans instituted new party rules in 1995, limiting committee chairs of their party to 6 years. Democrats took no comparable action.

To calculate seniority, the Senate gives credit for prior service in the Senate, in the House, or as governor of a state, in that order. The House only credits prior service in the House. Members within the same class, i.e. those sworn-in together on opening day, are ranked alphabetically.

A Member's service is deemed to begin at the start of the new Congress in which he/she is sworn into office. However, if a Member joins Congress to fill out an unexpired term, his/her length of service is computed in the House by the date of the special election to fill the vacancy. In the Senate, the official starting date is based on the date of the Senator's appointment, certification of election results, or the date of his/her swearing in as Senator.

In the Senate, seniority is still the determinant for the choice of committee chairs and ranking minority members on Senate committees. There have been no violations of the seniority system in the Senate for many years. However, in order to disperse power among more Senators, the two parties recently placed restrictions on how many committee leadership positions any one Senator may hold at the same time.

In the House, the top committee slots are effectively decided by the Republican and Democratic party leadership. The leadership's nominees are confirmed by a pro-forma vote of the entire party. Seniority has been breached numerous times in the House in the last 30 years, most recently when the Republicans took over the majority of the House in 1995. Three chairmanships of significant committees -- Appropriations, Judiciary, and Commerce -- were awarded to less senior Members of those committees. In addition freshmen Members were appointed to prestigious committees such as Appropriations, Ways and Means, and Rules. Traditionally, freshmen Members spent years earning the privilege of sitting on those key committees.

Seniority remains the basis for selecting larger, or more conveniently located, office space in both chambers. Members of the same "class" draw lots to decide remaining office space in a lottery process.

Party leadership selection in both chambers is not based on seniority. The position of Speaker and Majority Leader in the House and Majority and Minority Leader in the Senate are elected positions, competitively fought for within each party's caucus or conference, and decided largely on the basis of personal relationships and internal reputation among colleagues.