The US Congress failed to pass a stopgap funding measure to avert a government shutdown before a midnight deadline on Thursday, technically triggering the start of a shutdown.
What happens during US government shutdown? Here's all you need to know
The shutdown may be brief if the Senate and House of Representatives move quickly in the early morning hours of Friday, possibly ending before the start of the federal workday.
Overall: In shutdowns, non-essential government employees are often furloughed, or placed on temporary unpaid leave. Workers deemed essential, including those in public safety and national security, keep working, some with pay and others without.
After previous shutdowns, Congress passed measures to ensure essential and non-essential employees received retroactive pay.
A standoff over spending levels and immigration led to a three-day government shutdown, mostly over a weekend, in January. A shutdown in October 2013 lasted more than two weeks and more than 800,000 federal employees were furloughed.
Here is what happened in 2013, and what officials said in January could happen during a sustained shutdown:
Military: The Defense Department said in January a shutdown would not affect the US military's war in Afghanistan or its operations against Islamist militants in Iraq and Syria. All military personnel on active duty would remain on normal duty status. Civilian personnel in non-essential operations would be furloughed. Defense Secretary Jim Mattis said a sustained funding impasse would cause ships to go without maintenance and aircraft to be grounded.
Justice: The Justice Department has many essential workers. Under its shutdown contingency plan, about 95,000 of the department's almost 115,000 staff would keep working.
Financial oversight: The stock market-policing Securities and Exchange Commission funds itself by collecting fees from the financial industry, but its budget is set by Congress. It has said in the past it would be able to continue operations temporarily in a shutdown. But it would have to furlough workers if Congress went weeks before approving new funding. The Commodity Futures Trading Commission would have to furlough 95 percent of its employees immediately. They could be called back in the event of a financial market emergency.
National parks: National parks closed in 2013 and it resulted in a loss of 750,000 daily visitors, said the non-profit National Parks Conservation Association. The National Park Service estimated the shutdown resulted in $500 million in lost visitor spending in areas around the parks and the Smithsonian museums. Parks remained open in January.
Washington tourist sights: In 2013, popular tourist sites such as the Smithsonian closed, with barricades going up at the Lincoln Memorial, the Library of Congress and the National Archives. The National Zoo closed and its popular "Panda Cam" went dark. The Smithsonian remained open in January.
Taxes: The Internal Revenue Service furloughed 90 percent of its staff in 2013, the liberal Center for American Progress said. About $4 billion in tax refunds were delayed as a result, according to the Office of Management and Budget (OMB).
Mail delivery: Deliveries would continue as usual because the US Postal Service receives no tax dollars for day-to-day operations.
Travel: Air and rail travelers did not feel a big impact in 2013 because security officers and air traffic controllers remained at work. Passport processing continued with some delays.
Courts: The Administrative Office of the US Courts said in January federal courts, including the Supreme Court, could continue to operate normally for about three weeks without additional funding.
Healthcare: Sign-ups for the newly created Obamacare health insurance exchanges began as scheduled in 2013. The Medicare health insurance program for the elderly continued largely without disruption. A program at the US Centers for Disease Control and Prevention to track flu outbreaks was temporarily halted. Hundreds of patients could not enroll in National Institutes of Health clinical trials, according to the OMB.
Children: Six Head Start programs in Alabama, Connecticut, Florida, Georgia, Mississippi and South Carolina serving about 6,300 children shut for nine days in 2013, the OMB said.
Social security: Social Security and disability checks were issued in 2013 with no change in payment dates and field offices remained open but offered limited services. There were delays in the review process for new applicants.
Loans: Processing of mortgages and other loans was delayed in 2013 when lenders could not access government services such as income and Social Security number verification. The Small Business Administration was unable to process about 700 applications for $140 million in loans until the shutdown ended, the OMB said.
Veterans: Most employees at the Department of Veterans Affairs would not be subject to furlough. VA hospitals would remain open and veterans' benefits would continue, but education assistance and case appeals would be delayed, the department has said.
Food inspections: In 2013, Department of Agriculture meat inspectors stayed on the job. Agricultural statistical reports ceased publication. The USDA's website went dark.
Energy: The Department of Energy said in January that, since most of its appropriations are for multiple years, employees should report to work as normal during a shutdown until told otherwise. If there was a prolonged lapse in funding a "limited number" of workers may be placed on furlough.