President Pro Tempore of the Senate: History and Authority of the Office [Updated April 2, 2008] [open pdf - 189KB]
From the Summary: "The U.S. Constitution establishes the office of the President pro tempore of the Senate to preside over the Senate in the Vice President's absence. Since 1947, the President pro tempore has stood third in line to succeed to the presidency, after the Vice President and the Speaker of the House. Although the President pro tempore's powers are limited and not comparable to those of the Speaker of the House, as the chamber's presiding officer, he is authorized to perform certain duties. For example, he may decide points of order (subject to appeal) and enforce decorum in the Senate chamber and galleries. […] Since 1890, the President pro tempore has customarily been the majority party Senator with the longest continuous service. Twice, the Senate has also created an office of Deputy President pro tempore to honor a colleague, and an office of Permanent Acting President pro tempore in a third instance for the same reason. In 2001, the Senate also created an office of President pro tempore Emeritus. This report traces the constitutional origins and development of the office of President pro tempore of the Senate, reviews its current role and authority, and provides information on Senators who have held this office, and the more recently created subsidiary offices, over the past two centuries."
CRS Report for Congress, RL30960