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.

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.

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. 

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. 

The secretary of state is the fourth in line to the presidency. 

John J. Sullivan is the current acting secretary of state. President Trump has nominated former CIA Director Mike Pompeo for the role. 

The Presidential Succession Act says those in the line of succession must be appointed "by and with the advice and consent of the Senate." Often, acting cabinet members have been confirmed by the Senate in some role, just not the one they're temporarily filling. The language is unclear about whether those in acting positions are eligible for the line of succession. 

The secretary of the treasury is the fifth in line for the presidency.

Steven Mnuchin has served in that role since February 2017. 

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. 

The attorney general is the seventh in line for the presidency. 

Jeff Sessions currently serves in that role. 

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. 

The secretary of agriculture is ninth in line for the presidency.

Sonny Perdue currently serves in that role. 

The secretary of commerce is tenth in line for the presidency. 

Wilbur Ross currently serves in that role. 

The secretary of labor is eleventh in line for the presidency

Currently, Alexander Acosta serves in that role. 

The secretary of health and human services is twelfth in line for the presidency. 

Alex Azar currently serves in that role. 

The secretary of housing and urban development is thirteenth in line for the presidency. 

Ben Carson currently serves in that role. 

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. 

The secretary of energy is fifteenth in line for the presidency.

Rick Perry currently serves in that role. 

The secretary of education is sixteenth in line for the presidency. 

Betsy DeVos currently serves in that role. 

The secretary of veterans affairs is seventeenth in line for the presidency.

Robert Wilkie is the acting secretary of veterans affairs following the dismissal of David Shulkin. Trump's pick for the role is presidential physician Ronny Jackson.

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.