Buddhist contemplative traditions have thrived in Tibet since at least the seventh century CE, and have taken an astonishing variety of forms ranging over the entire spectrum of Indian and Central Asian Buddhist traditions. This diversity is usually organized under the rubric of "three vehicles" in Buddhism—the Lesser (Hīnayāna), Great (Mahāyāna), and Adamantine (Vajrayāna) vehicles. The historical challenge was to integrate this diversity into cogent systems of practice, and especially how to integrate exoteric Buddhist contemplation based on canonical sūtras, and esoteric forms of Buddhist meditation derived from canonical Tantras. Most Tibetan traditions came to see the Tantric methods as intrinsically superior in their capacity to generate more rapid realization due to their directness. By the thirteenth century, Tibet had established itself as the international center of esoteric Buddhism, and alone developed the full spectrum of Buddhist...