In the early 1930s, a wave of Marian apparitions – cases in which visionaries reported seeing and receiving messages from the Virgin Mary – swept over Belgium. With over forty apparition sites and hundreds of visionaries, the Belgian apparitions, often attended by crowds of onlookers, were unrivalled in scope and complexity, and they confronted Catholics and others with a question: How do you decide what you believe? Apparition Fever explores the Belgian apparitions from initial reports to the eventual recognition of two episodes in the 1940s. It shows how knowledge was formed at all levels – among the bystanders attending the sites, to the medical experts who studied the visionaries, and the clerical authorities who evaluated the authenticity of the apparitions. Tine Van Osselaer examines how these different perspectives converged and influenced each other, whose authority was accepted or challenged, and how the public character of the events affected their evaluation. Apparition Fever reveals that the public setting of Marian apparitions and the religious enthusiasm they triggered are not novel challenges for the Catholic Church. On the contrary, they have heavily influenced the evaluation of apparitions since the early twentieth century.