The fight against fake news or disinformation mobilizes the press, NGOs, legislators and the major Internet companies. Manipulation is said to be responsible for irrational voting (Brexit, Trump), for the loss of confidence in democracy, and for the soon-to-be advent of the post-truth. For some, the Yellow Vests are the puppets of Russian trolls and extremists. Can we measure the impact of manipulation? It is no longer only done through the mass media; it is becoming more democratic through social networks. The gap is widening between elites for whom only manipulated or delusional people can doubt the obvious and populations impervious to discourse from above. Everyone finds, in the media or on the Internet, something to confirm their beliefs. At a time when citizens can no longer even debate a common reality, the book points out the technological, social, cultural and ideological causes. It explains the methods for spotting fakes, verifying, denouncing and countering them. It also shows the perverse effects. It prepares us for one of our worst challenges: falsification, doubt and mind control in the public space.