Mount Atago lies northwest of the ancient capital of Kyoto. Because of its long history and the cult that has been associated with it from early times, the mountain provides a typical example of Japanese religion. Like all sacred mountains in Japan, a cult of ancestors was originally attached to Mount Atago. During successive centuries this cult served as a base for other cults: of fire, the tengu, the Bodhisattva Jizō, and Shōgun Jizō. It also served as the base for a large popular cult with branches all over the country, which still exists. The complex structure of the popular cult contains a harmonious blend of elements of archaic religion, Shinto, and Buddhism. From early times until the Meiji period, its organization was directed by a group of the mountain ascetics known as yamabushi, who lived on the mountain itself. A sad consequence of the Meiji Restoration was the dispersion and disappearance of this group as well as most of the documents concerning Mount Atago. In an effort to reconstruct the history of the cult, the writer has consulted the few documents that still remain, which are found among local chronicles and classical texts. The study also discusses the religious and social characteristics of the Atago confraternities (kō), which are found in towns and villages even today, and their position in relation to the general phenomenon of confraternities in Japan.