With the available historical Tibetan written records from late 8th century on and the existing scholarly works on Buddhism, this historical overview recounts how Buddhism was Tibetanized and how it became both the national religion of Tibet and a world religion spread to Inner Asia, East Asia, and other parts of the world. It also adds interpretive commentaries leading to more historical inquiries and suggestions for alternative historiographical approaches to the formation of Tibetan Buddhism, adopted from disciplines other than history of religion and Buddhist studies. An emphasis is placed on the significance of folk accounts that reveal “the geomythological reorientation” of Buddhist conversion in the historical Tibetan context not merely as an intellectual and doctrinal acceptance of Indian Buddhism but also as a symbiotic process in which Indian Buddhism and indigenous religious practices mutually transformed each other. The emergence of the different Buddhist schools in Tibet is also a result of the politics of the sect-specific powers throughout Tibetan history. It is thus essential to recognize the formation of the five schools also as a set of religio-political occurrences, particularly since the formation of Gelug (dGe lugs) School in the 15th century and later becoming a Gelug-based Tibetan polity in the 17th century. The Gelug School dominated Tibetan Buddhism, and successive Dalai Lamas ruled Tibet from the mid-17th to the mid-20th centuries. Given the regional and global status of Tibetan Buddhism, emphasis is placed on Tibetan Buddhism as a transregional religion in Inner Asia and later as a form of modern Buddhism since the middle of the 20th century. With these emphases, the historical overview presented here is intended to generate more scholarly discussions and inquiries into the history of Tibetan Buddhism in both monastic and lay spheres in and outside Tibet.