Conspiracy theories are on everyone's lips and are the subject of many thought experiments, ideologies, and fictional works. But how widespread are the ideas about dark world domination plans, murdered superstars, or covered-up alien landings? And who believes them? Is there a typical conspiracy theorist? What do they look like? And why do people believe in even the most absurd theories? In the course of his doctoral thesis, psychologist and journalist Sebastian Bartoschek explored these and many other questions. Over three years, he examined 95 different conspiracy theories and surveyed several thousand people in total. What emerged is the world's most comprehensive study of conspiracy theories, supported by empirical data that is as fascinating in its own right as it is in terms of future research possibilities, alongside a quantitative analysis of the topic.