What, if anything, should be banned from online media? And who should review violent and explicit content, in order to decide if it’s okay for the public? Thousands of people around the world are working long, difficult hours as content moderators in support of sites like Facebook, Twitter, and YouTube. They are guided by complex and shifting guidelines, and their work can sometimes lead to psychological trauma. But the practice of content moderation also raises questions about censorship and free expression online.