Hundreds of people have complained about WhatsApp being down, with many unable to send or receive photographs.
Users around the world reported problems with the Facebook-owned messaging app on Sunday morning.
According to website health tracker Down Detector, Europe is the worst affected region.
Parts of Asia and South America were also affected.
Dozens of people took to Twitter and Facebook to express their frustration.
The main problem appeared to be sending and receiving photographs and other media, including stickers.
There were dozens of reports in the UK.
By 2.30pm, there were still reports of trouble with the app, but many people seemed to have full service again.
During an outage in March, users were cut off for almost a whole day.
More than 500 million people use the app each day.
WhatsApp has not commented on the issue.
