The Presidential Succession Act is the order of who becomes President in the event the President dies or becomes unable to perform his duties. If catastrophe struck and lots of high-up people died, who becomes what? There were PSAs in 1792, 1886, 1947, and a recent addition had the Secretary of Homeland Security added to the list. There's an excellent article about it here. The Act has an interesting history, and the article I linked to (Wikipedia) has a few things most of us don't think about or know. I kinda went "Wow!" with the following:

"Presidents who became president by succession

Chester A. Arthur
Calvin Coolidge
Millard Fillmore
Gerald R. Ford *
Andrew Johnson
Lyndon B. Johnson
Theodore Roosevelt
Harry S. Truman
John Tyler"

(The aztrix/atorix/astiricks/ star indicates that Ford became President upon Nixon's resignation; all the rest because the President had died.)