Lunacy, the legendary notion of minds unhinged by the moon, continues to captivate the popular imagination. Although it violates the assumptions of modern science and psychiatry, such belief remains common among mental health workers. Furthermore, several studies have found a small, unexplained correlation between behaviour and the lunar cycle. The book is divided into two parts. It begins with a historical account of the lunacy concept, followed by an investigation of hypothetical mechanisms for a lunar effect.