Presidential Succession: An Overview with Analysis of Legislation Proposed in the 109th Congress [June 29, 2005] [open pdf - 123KB]
"Whenever the office of President of the United States becomes vacant due to 'removal ... death or resignation' of the chief executive, the Constitution provides that 'the Vice President shall become President.' When the office of Vice President becomes vacant for any reason, the President nominates a successor, who must be confirmed by a majority vote of both houses of Congress. If both of these offices are vacant simultaneously, then, under the Succession Act of 1947, the Speaker of the House of Representatives becomes President, after resigning from the House and as Speaker. If the speakership is also vacant, then the President Pro Tempore of the Senate becomes President, after resigning from the Senate and as President Pro Tempore. If both of these offices are vacant, or if the incumbents fail to qualify for any reason, then cabinet officers are eligible to succeed, in the order established by law (3 U.S.C. §19, see Table 3). In every case, a potential successor must be duly sworn in his or her previous office, and must meet other constitutional requirements for the presidency, i.e., be at least 35 years of age, a 'natural born citizen,' and for 14 years, a 'resident within the United States.'"
CRS Report for Congress, RL32969
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html