It started with a few bare-faced ladies and then grew very rapidly. Even men have got involved by doing the complete opposite.
Now, in less than a week, ladies' make-up free selfies, and men's make-up selfies, have raised more than 8 million pounds for Cancer Research UK.
Lots of the money will go to the Cambridge Institute, one of the main research bases of cancer research UK, where they're coming up with new ways of diagnosing, monitoring and treating cancer.
The pioneering work they do here's been given a boost, simply by people posting selfies onto social media websites and making a donation - then encouraging friends and family to do it too.
_"I think it's absolutely fantastic, the no make-up selfie, women taking photos of themselves with no make up on. The money from that is going to fund work here and also throughout the whole of Cancer Research UK. I've done my own selfie and apparently it's not as bad as it could have been."
Hayley Whitaker, Scientist
Great grandmothers May and Brenda from Norfolk have joined the craze. They're thought to be the oldest ladies to take part in the fundraising campaign.