In the mountain valleys of Nepal, Tibetan communities have long been established through migrations from the North. Because of these migrations over the last few centuries, Tibetan lamaism, as one of the world's great ritual traditions, can be studied in the Himalayas as a process that emerges through dialogue with the more ancient shamanic tradition which it confronts and criticizes. Here for the first time is a thorough anthropological study of Tibetan lamaism combining textual analysis with richly contextualized ethnographic data. The rites studied are of the Nyingma Tibetan Buddhist tradition. In contrast to the textual analyses that have viewed the culture as a finished entity, here we see an unbounded ritual process with unfinished interpretations. Mumford's focus is on the dialogue taking place between the lamaist and the shamanic regimes, as a historic development occurring between different cultural layers. The study powerfully demonstrates that interrelationships between subsystems within a given cultural matrix over time are critical to an understanding of religion as a cultural process.