Some GPs in Wales have been running at limited capacity without computer systems following a global IT outage.
Planes have been grounded throughout the world, many industries disrupted and TV channels forced off air.
In Wales, trains have been delayed or cancelled, GPs are having to revert to handwritten notes and are turning away all but the most serious patients.
The exact cause of the problem is unknown, although several firms have blamed an issue with Crowdstrike cyber security software.
Llan Healthcare, a GP surgery in Llanrumney, Cardiff, wrote on Facebook: “We are currently unable to access our clinical system.
“We are running on a limited capacity until we know more.
“Digital Healthcare Wales are aware of the situation and are working to try get the issue resolved.
“Please only phone if it is an emergency you can utilise 111 and attend local pharmacies.”
A spokesperson for Transport for Wales said “some” IT systems are currently impacted, but that services are “running as planned”.
“Customers are advised to check for the latest information before travelling. We apologise for any inconvenience caused.”
Cardiff Airport has said the IT issues are having “minimum impact” on operations at present.
“Passengers who are due to fly from Cardiff Wales Airport today are advised to check our website for live flight information,” said a spokesperson.