If the president and vice president can not serve who serves?

If one or both cannot serve because they are temporarilyincapacitated, the 25th Amendment to the United States Constitution prescribes who will become the Acting President. If the president and vice president are permanently removed because of death, resignation, etc., then the Speaker of the House of Representatives becomes the President of the United States as prescribed by the Constitution of the United States and the Presidential Succession Act of 1947.