Section 1 of the 25th Amendment
Text of Section 1:
"In case of the removal of the President from office or of his death or resignation, the Vice President shall become President."
The provisions of Section 1 to date have been implemented but once: upon the resignation of Richard Nixon on August 9, 1974. Click here for more details.
Author's Notes and Comments
Thanks to the ambiguous, original language written into Article II, Section 1, Clause 6 of the Constitution, for the first 64,991 days of the United States Government's history, it was not clearly and unequivocally stated that a Vice President automatically succeeded to the presidency.
The "Tyler Precedent" of 1841, followed by the successions of Millard Fillmore, Andrew and Lyndon Johnson, Chester Arthur, Theodore Roosevelt and Harry S Truman each had set it as a de facto standard, but until ratification of the 25th Amendment, the constitutionality of succession remained somewhat in question.