During his nine-month stay in China, the Japanese monk Saicho (767-822) was alleged to have been initiated into an illustrious esoteric lineage starting from a prestigious Indian Esoteric patriarch. It is also believed that Saicho's Chinese mentor, based on three esoteric texts translated by the Indian Patriarch, transmitted to him some particular forms of esoteric teachings, the core of which is preserved in one of the two "dharma-transmission documents" from Saicho's teacher. This is the conventional view regarding the roots of Tendai Esoteric Buddhism in Japan. This book subjects this conventional view to a critical examination. It argues that the two "dharma-transmission documents" were not written by Saicho's Chinese mentor, but were prepared in Japan for strengthening the legitimacy of the initiation Saicho received from China. The three siddhi texts attributed to the Indian patriarch were also composed in Japan as the scriptural support for Saicho's esoteric transmission. The Tendai form of Esotericism in the name of Saicho was for the main part created not by Saicho himself but by his followers.These negative conclusions can be turned into a positive agenda for future research of Japanese Tendai Buddhism. Scholars can turn from a fruitless search for the roots of Tendai Esotericism in China to look more closely in Japan. This study may invite more scholarly attention to a host of Buddhist apocrypha which, long regarded as Chinese, were actually produced in Japan or Korea. This book also addresses issues of larger implications for East Asian religions, including the manufacturing process of the so-called Buddhist apocrypha and their roles, lineage construction for the purpose of religious legitimation and of overcoming borderland complexes, application of overarching theoretic patterns in different religious traditions.