This book presents a comprehensive study of the Abhayagiri Fraternity of ancient Sri Lanka, with a special reference to its new Buddhist practices and trends, which made a great contribution to the development of Buddhism in Sri Lanka. It involves a consideration of both primary and secondary literary sources, and also takes into account archaeological findings, epigraphic material and liturgical artifacts. It contains a detailed examination of how the Abhayagiri School adopted heterodoxy Buddhism of other Buddhist traditions apart from the orthodox Theravada teachings and practices. The Mahavihara and the Abhayagiri both accepted the Pali Tipitaka as authoritative texts, but the Abhayagiri went a further step by accepting some non-Theravada teachings including Vaitulyavada (Mahayana) and Vajiriyavada (Vajrayana), in marked contrast to the Mahavihara ideology. A number of political, social, historical and doctrinal issues caused for this changing. Although the Abhayagiri School developed a number of new doctrinal and philosophical ideas, this work mainly focuses on the practical issues rather than its philosophical aspects.