For facial recognition technology to work in the public sphere, you need two things: really good software and lots of cameras. Much has been made of China's use – and export – of facial recognition technology for security and policing purposes, but when it comes to Big Brother-style surveillance of public spaces, plenty of European and American cities have a huge number of eyes in the sky too. As these cities deploy facial recognition software with increasing frequency, the ethical and legal battles are only just beginning.