A mass IT outage has caused chaos around the world, with major banks, media outlets and airlines among the victims.
Many flights have been grounded, with queues and delays at airports, while shops and communications have also been hit.
The cause is unknown, but Microsoft says it is taking "mitigation action" to deal with "the lingering impact" of the outage.
Australia's home affairs ministry and American Airlines have both said the outage appeared to be related to an issue at global cyber-security firm Crowdstrike.
Here is a summary of what we know so far.
What caused this?
What precisely went wrong is still unclear.
However, a spokesperson for Australia's Home Affairs Minister said the outage appeared to be related to an issue at global cyber-security firm
Crowdstrike.
Australia's National Cyber Security Coordinator described it as a "large-scale technical outage" and said there was no information to suggest it was an attack.
American Airlines, which is the world's biggest by passenger numbers, told the BBC the IT problems were because of a "technical issue with Crowdstrike that is impacting multiple carriers."
United Airlines told the BBC: "A third party software outage is impacting computer systems worldwide, including at United."
There have been reports suggesting that Crowdstrike, which produces antivirus software, issued a software update that caused Windows devices to crash.
Crowdstrike shares plunged as much as 14% in early trade, and Microsoft also declined, as did travel and leisure stocks, as the disruptions threaten the holiday season.
The BBC has approached Crowdstrike for comment but has not yet heard back.
What exactly is Crowdstrike?
It's a reminder of the complexity of our modern digital infrastructure that Crowdstrike, a company that's not exactly a household name, can be at the heart of such worldwide disarray.
The US firm, based in Austin, Texas, is a listed company on the US stock exchange, featuring in both the S&P 500 and the high-tech Nasdaq indexes.
Like a lot of modern technology companies, it hasn't been around that long. It was founded a mere 13 years ago, but has grown to employ nearly 8,500 people.
As a provider of cyber-security services, it tends to get called in to deal with the aftermath of hack attacks.
It has been involved in investigations of several high-profile cyber-attacks, such as when
Sony Pictures had its computer system hacked in 2014.
But this time, reportedly because of a flawed update to its software, a firm that is normally part of the solution to IT problems may instead have a role in their origin.
Who is affected?
The problem emerged in a piecemeal fashion, with first reports coming out of
Australia.
Payment systems went down at Australian stores including
Woolworths, while financial institutions such as the
National Australia Bank were also affected.
Then the issues spread to the
US. The state of
Alaska warned that its emergency services were affected, while several of the country's airlines -
United,
Delta and
American Airlines - grounded their flights around the globe.
Australian carriers
Virgin Australia and
Jetstar also had to delay or cancel flights as departure screens went blank at Sydney airport.
Tokyo-Narita and
Delhi airports said services had been affected. European airports reported that the outage was causing delays, with long queues reported at London's
Stansted and
Gatwick and Amsterdam's
Schiphol.
Ryanair said it was experiencing "potential disruptions across the network", which it said were due to a third-party outage.
Broadcasters were also caught up in the chaos, including
Sky News in the UK, which is off air.
As the full extent of the disruption became clear, more firms and institutions started reporting problems.
The
London Stock Exchange said it was working as normal, but there were issues with its news service, used by companies to report market-sensitive information in a timely way.
Israel says 15
hospitals have switched to manual processes but this has not affected medical treatment. Ambulances are being told to take new cases to other hospitals.
And Poland’s largest container terminal, the
Baltic Hub in the northern city of
Gdansk, has said the outage is "hampering terminal operations" and has asked companies not to send containers to the port.
In the UK, railway companies have reported delays and said they were experiencing "widespread IT issues", while some doctors' surgeries in England reported issues with booking appointments.
The problems have extended to upmarket UK bakery chain
Gail's, which has said it is currently unable to take in-store payments.
: I wish you a good and safe flight!
