The US Federal Aviation Administration has lifted the longest nationwide ground stop of flights since the 11 September 2001 attacks.
The FAA said Friday's stoppage, which lasted more than four hours, was "due to extraordinary circumstances" from a communication issue affecting all air traffic control centres.
Homeland Security Secretary Markwayne Mullin said there was no indication of a cyber-attack and that everybody was safe.
Here's what we know.
The ground stop led to more than 8,500 delayed and nearly 8,000 cancelled flights on Friday, according to FlightAware.
The FAA's initial announcement at 15:54 EST (20:54 GMT) said all flights departing within the US would be grounded until further notice.
More than 3,500 airplanes were in the air at the time. Some were diverted to other airports and others flew on to their destinations.
Hours later, some flights were allowed to take off from San Francisco International airport.
The Federal Aviation Administration opened skies again at until 20:15 EST.
All US-bound air traffic from Eurocontrol member states was suspended.
There are 42 Eurocontrol member states, including dozens of EU nations, the UK and Turkey, as well as Israel and Morocco.
Passengers travelling from the UK have been warned of possible disruption on Saturday morning.
A number of flights to the US from Heathrow airport were cancelled and delayed, but Manchester airport said no significant disruption was expected. Flights were operating as normal from Gatwick airport.
Australia's largest international airport, Sydney, had grounded all US-bound flights, but scheduled flights are now expected to depart as planned.
Air Canada also cancelled flights between the US and Canada.
US Transportation Secretary Sean Duffy said in a news conference on Friday night that the outage would cause a ripple effect for a few days.
"It's going to take us some time to unwind what's happened this afternoon," Duffy said.
Tens of thousands of people were stranded in airports around the country, many of them "without any knowledge of where they're going to sleep tonight", Duffy said.
Delta Air Lines and United Airlines said they were resuming operations.
American Airlines said it was "focused on operating as many remaining flights as possible this evening". JetBlue said it would resume flights on Saturday morning.
British Airways, which has had a few US-bound flights affected, advised passengers to check their flight status but to travel to the airport as planned.
In the US, UK and the EU, airlines may be liable for compensation if the reason for a delay is deemed within their control, such as a fault with the plane or pilot sickness.
As such, airlines do not have to provide compensation to passengers for air traffic control problems.
But passengers are still protected to a certain degree under US federal - and country-specific - laws.
Under the FAA Reauthorization Act of 2024, cancelled or "significantly delayed flights" - defined as three hours for domestic flights and six hours for international routes - must be offered alternative travel or a full refund of the fare if the passenger chooses not to fly.
It also requires all air carriers to issue refunds for tickets purchased with credit cards within seven business days of the request, and 20 days for tickets purchased with another form of payment. Passengers accepting a new flight cannot receive a refund.
In the US, there is no requirement for airlines to provide meal vouchers or accommodation, but some may offer complimentary support to customers.
However, if passengers are travelling from the UK to the US or flying with a UK carrier, you may have additional rights.
Under UK law, airlines have a duty to look after you when flights are delayed or cancelled for whatever reason.
This includes:
a reasonable amount of food and drink (often through vouchers)
a way for you to communicate (often by refunding the cost of calls)
free accommodation, if you have to stay overnight to fly the next day
transport to and from that accommodation
If your airline is unable to arrange assistance, you have the right to organise this yourself and claim back the cost later.
The Civil Aviation Authority advises people to keep receipts and not spend more than necessary.
It was only the second such nationwide stoppage since the immediate aftermath of the al-Qaeda plane hijackings of 11 September 2001.
"This looks like the worst since 9/11, but clearly not on that scale," Dr Guy Gratton, an associate professor of aviation at Cranfield University, told BBC Verify.
Gratton pointed to a US grounding in 2023 due to a failure in the Notice to Air Missions (Notam) system.
"That lasted 1 hour and 52 minutes, delayed about 9,500 flights and cancelled about 1,300," he said.
The 2023 outage meant air traffic safety notices could no longer be published and flights were grounded until that system was restored.
