WHAT'S OPEN AND WHAT'S CLOSED
Social Security checks will go out and troops will remain at their posts. Doctors and hospitals will receive their Medicare and Medicaid reimbursements. The U.S. Postal Service is an independent agency and won't be affected. Passport services, which are funded by fees and not government spending, will also continue.
Virtually every essential government agency, including the FBI, the Border Patrol and the Coast Guard, will remain open. Transportation Security Administration officers will staff airport checkpoints.
The air traffic control system, food inspection, Medicare, veterans' health care and many other essential government programs will run as usual. The Federal Emergency Management Agency and Environmental Protection Agency can continue to respond to disasters.
Nearly 90 percent of the Department of Homeland Security's 240,000 employees will be at work because they're considered essential.
Special counsel Robert Mueller's office, which is investigating potential ties between the Trump campaign and Russia, is unaffected by a shutdown. But in New York, the chief judge of Manhattan federal courts has suspended work on civil cases involving U.S. government lawyers, including several civil lawsuits in which Trump himself is a defendant.
As hundreds of thousands of federal workers are forced off the job, some services will go dark.
Farmers hardest hit by the trade war with China may have to wait for a second round of direct payments from the Agriculture Department, and new farm loans will be put on hold.
In the past, the vast majority of national parks were closed to visitors and campers, but beginning with the last government shutdown, in January, the Interior Department has tried to make parks as accessible as possible despite bare-bones staffing levels. Some are staying open thanks to funding from states and charitable groups, but others are locked off.
In Washington, popular museums, art galleries and the National Zoo have remained open by using unspent funds, but the money is running out and they will close starting midweek if the shutdown continues.
FEDERAL WORKERS STILL GET PAID — EVENTUALLY
While they can be kept on the job, federal workers won't be paid for days worked during the lapse in funding. In the past, however, they have been repaid retroactively even if they were ordered to stay home. The Senate already has passed legislation ensuring that workers will receive back pay. The House seems sure to follow suit.
But government contractors won't get paid for the time they lose staying home, causing problems for those who rely on hourly wages.
Federal workers are exempted from furloughs if their jobs are national security-related or if they perform essential activities that "protect life and property."
Roughly 420,000 workers were deemed essential and are working unpaid, unable to take any sick days or vacation, including about 40,000 law enforcement and corrections officers. The Homeland Security employees who will keep working include about 150,000 from the Coast Guard, TSA and Customs and Border Protection.
An additional 380,000 are staying home without pay. They include nearly all from NASA and Housing and Urban Development and about 40,000 from the Commerce Department. About 16,000 National Park Service employees — 80 percent of the agency's workforce — are furloughed.
Also among those furloughed are 52,000 staffers at the Internal Revenue Service, slowing analysis and collection of hundreds of thousands of tax returns and audits.
Shutdowns happened every year when Jimmy Carter was president, averaging 11 days each. During President Ronald Reagan's two terms, there were six shutdowns, typically just one or two days apiece.
Before a three-day lapse in January, caused by Democrats' insistence that any budget measure come with protections for young immigrants known as "Dreamers," the most recent significant shutdown was a 16-day partial shuttering of the government in 2013.
That one came as tea party conservatives tried to block implementation of President Barack Obama's health care law. The government also shut down for a few hours last February amid a partisan dispute over deficit spending.