Rudolf Otto exerted a major formative influence on the study of religions in the first half of the 20th century. His landmark book, Das Heilige (1917), translated into English as The Idea of the Holy (1923), identified as central to religion the experience of the numinous, a word he coined. That numinous was, in his terms, a mysterium tremendum et fascinans-a fascinating yet terrifying mystery. This influential view set the agenda for most professional scholars of religion until well into the 1960s. It also introduced a set of concepts that has enjoyed wide cultural currency. Still today creative writers and artists occasionally refer to the numinous. This book introduces general readers to Otto's thought in The Idea of the Holy and helps them develop a fuller understanding of it. It presents the problems with which Otto wrestled, among them questions about evolution and religion, the historical development of Christianity, and religion's role in making a better society. It sets Otto's ideas about the numinous in the broader currents of thought to which they belong. It also examines criticisms of Otto's ideas and concludes with an examination of how scholars today approach the topics Otto addressed.