Sophisticated digital imaging technology is being used to create 'Deep Fakes'; images and videos that are indistinguishable from reality. Things have moved on from celebrity faces being superimposed onto porn performers. As the slurring Nancy Pelosi video shows, these fakes can now have a political agenda. Professor Hany Farid of UC Berkeley specialises in detecting the signs of digital manipulation, and is trying to design systems that can tell the real from the fake before things go viral. But with hundreds of hours of uploaded video hitting YouTube's servers every minute, it's an all-but-impossible task: and the stakes are high if things get missed.