If the President dies, resigns or is removed from office, the vice president takes over. But if the vice president can't assume the role, who's next?
The Presidential Succession Act was passed in 1772, but the line of succession as we know it today was solidified in the Presidential Succession Act of 1947. And the 25th Amendment, ratified in 1967, specifically allows the vice president to temporarily assume the presidency in case the president falls ill or is, for a time, unable to carry out the duties of the office.
Here's who's in line after President Donald Trump.
1. Vice President
The vice president assumes the role of the presidency if the president dies, resigns or is impeached. The vice president also fills in if the president cannot fulfill his or her duties.
Vice President Mike Pence is currently first in line.
First in line
Eight vice presidents in history have assumed the presidency after the death of the president and one after a president's resignation.
When there is a vacancy in the office of vice president, the president nominates someone who must be confirmed by a majority in both houses of Congress.
2. Speaker of the House
The speaker of the house assumes the role of the presidency if something happens to both the president and vice president. This has never happened.
Currently, Paul Ryan is second in line for the presidency. Ryan announced in April 2018 that he would not be running for reelection, so the great responsibility will go to someone else come January 2019.
3. President Pro Tempore of the Senate
The president pro tempore, the longest serving senator from the majority party in the Senate, is third in line to the presidency.
Currently, Sen. Orrin Hatch, R-Utah, is the president pro tempore of the Senate. Hatch announced in 2018 that he would retire after serving seven terms in the Senate.
4. Secretary of State
The secretary of state is the fourth in line to the presidency.
5. Secretary of the Treasury
The secretary of the treasury is the fifth in line for the presidency.
Steven Mnuchin has served in that role since February 2017.
6. Secretary of Defense
The secretary of defense is the sixth in line for the presidency.
James Mattis, a retired United States Marine Corps general, currently serves in that role.
7. Attorney General
The attorney general is the seventh in line for the presidency.
Jeff Sessions currently serves in that role.
8. Secretary of the Interior
The secretary of the interior, who oversees the Bureau of Land Management, the United States Geological Survey, and the National Park Service, is eighth in line for the presidency.
Ryan Zinke currently serves in that role.
9. Secretary of Agriculture
The secretary of agriculture is ninth in line for the presidency.
Sonny Perdue currently serves in that role.
10. Secretary of Commerce
The secretary of commerce is tenth in line for the presidency.
Wilbur Ross currently serves in that role.
11. Secretary of Labor
The secretary of labor is eleventh in line for the presidency
Currently, Alexander Acosta serves in that role.
12. Secretary of Health and Human Services
The secretary of health and human services is twelfth in line for the presidency.
Alex Azar currently serves in that role.
13. Secretary of Housing and Urban Development
The secretary of housing and urban development is thirteenth in line for the presidency.
Ben Carson currently serves in that role.
14. Secretary of Transportation
The secretary of transportation is fourteenth in line for the presidency.
Elaine Chao currently serves in that role, but because she is not a natural-born citizen, she is not eligible to serve as president. Chao was born in Taiwan. She is the first Asian-American woman to be appointed to a U.S. president's cabinet.
15. Secretary of Energy
The secretary of energy is fifteenth in line for the presidency.
Rick Perry currently serves in that role.
16. Secretary of Education
The secretary of education is sixteenth in line for the presidency.
Betsy DeVos currently serves in that role.
17. Secretary of Veterans Affairs
The secretary of veterans affairs is seventeenth in line for the presidency.
18. Secretary of Homeland Security
The secretary of homeland security is eighteenth and last in line for the presidency.
Kirstjen Nielsen currently holds that position.
During the State of the Union address, one person in the line of succession is chosen as the "designated survivor" and stays behind at a secure location. But what would happen in the event of a catastrophe that left everyone in the line of succession dead?
After Sept. 11, the idea of a "First Secretary," an official whose job it would be to stay out of Washington and be first in line to the presidency after the vice president, was proposed. But it never came to be. So in the case of a an all-out American nightmare, there's no further contingency plan.