When Socrates was standing before the Athenian tribunal in 399 BC, he said in his defence that the opponents he feared most were the invisible ones, those who had been spreading rumors against him for years but none of whom were being brought to court - it was like fighting shadows. The moment was iconic: Socrates, the harbinger of logos and true knowledge, was eventually defeated by rumors and mendacious slander. Where does the strange power of rumors come from? Everyone knows that rumors are unfounded and based on thin air, but still they pass them on: rumors spread, and what appeared as a small breeze can grow into a mighty whirlwind and produce serious effects, ruin people's lives and change the course of events. This book scrutinizes the mysterious power of rumors and seeks to analyse it philosophically, examining along the way some key moments of our cultural history concerning rumors, from Shakespeare and Cervantes to Gogol and Kafka. It also underlines the fact that, although rumors are as old as humankind, the advent of the internet and social media has raised the spreading of rumors to an entirely new level, to the point where we could speak of the rumorization of the social. The more communication there is, the more the social fabric threatens to fall apart - and the more urgent it becomes to find strategies to counteract this.