The government has officially shut down 18 times since 1976, when the current federal budgeting process was instituted.
The last time was in 2013, in a deadlock over health care policy. The shutdown lasted 16 days and furloughed hundreds of thousands of federal workers.
What stops and what continues during a federal shutdown varies, but in 2013, 850,000 federal workers were furloughed, meaning they could not come to work. Technically, federal workers cannot be paid for those days, but in the past, they have been paid retroactively.
The 850,000 figure amounts to less than half of the federal civilian workforce of 2 million. Essential agencies, such the FBI, Border Patrol, and Voice of America, continue functioning with a skeleton staff. Air traffic controllers will stay on the job, as will federal security agents at airports.
Overseas, U.S. embassies also have "essential" staff members who will continue to perform basic duties; however, the State Department has not elaborated on what duties it could still perform in the event of a shutdown.
U.S. Secretary of State Rex Tillerson has said his department is "ready" if the government shuts down. Department spokeswoman Heather Nauert told reporters Thursday that officials had yet to decide what services will continue but added, "We will be prepared for all contingencies."
In 2013, immigration and citizenship services continued, but were limited. The U.S. Electronic Immigration System, which includes an "e-verify" system to help process employment applications, is expected to be unavailable during a shutdown.
U.S. mail services are expected to continue, but federal tax refunds could be delayed. White House budget director Mick Mulvaney said Friday that the national parks would be open this time, especially if services are provided by third parties. Washington, D.C. Mayor Muriel Bowser told reporters Friday the city will pick up the trash all around the monuments on the National Mall and bill the federal government.
The National Zoo would likely close to visitors, although workers would continue to feed and care for its residents — some 1,800 animals of about 300 different species.
Based on 2013, federal courts can be expected to remain open. The Administrative Office of the U.S. Courts has said the federal court system can function for about three weeks without needing additional funds.
Medicare insurance for the elderly is expected to continue, but research programs at the U.S. Centers for Disease Control and Prevention could be suspended until funding is restored.
Military personnel are expected to continue working, but civilian employees of the military would likely be placed on unpaid leave.
The Veterans Administration is expected to continue functioning, including operation of its hospitals.
Some agencies, like the federal courts and Department of State, can function for several weeks on their remaining funds. After that time, more services could be curtailed.