Many accounts place the origins of Tantric Buddhism in Japan in the hands of the two men, Saichō 最澄 (767–822) and Kūkai 空海 (774–835). (This article will use “Tantric” and “esoteric Buddhism” synonymously.) These were the founders, respectively, of the Tendai (天台) and Shingon (真言) schools, both of which contributed substantially to the early development of Japanese forms of Tantric theory and practice. Naturally, no tradition emerges from a vacuum; it always grows from existing roots and trunks to create new branches. Because the contributions of Saichō and Kūkai marked a major transition in the history of Japanese Buddhism, focusing on them is an appropriate way to frame important features of early Tantrism in Japan. Several of the deities central to developed esoteric Buddhism in Japan were present during the Nara period (710–794), as were some of the key texts such as the Scripture of the Great Illuminator大日経 (Skt. Mahāvairocana-sūtra, Jpn. Dainichi-kyō), prior to Saichō and Kūkai’s bringing new materials back from China in 805 and 806, respectively. Significant among the new elements were mandalas, initiation or consecration ceremonies (kanjō灌頂) into ritual practice that employed them, and new texts, in particular of the Scripture of the Tip of the Thunderbolt (金剛頂経) (Skt. Vajraśekhara-sūtra, Jpn. Kongōchō-kyō) corpus, most of which had been translated into Chinese by Amoghavajra 不空 (705–774, Ch. Bukong; J. Fukū). While Saichō returned to Japan more than a year before Kūkai—and established the earliest foundation for the new Tantric tradition by performing Japan’s first kanjō and by making one of the two formal tracks for training Tendai monks a Tantric one (shana-gō遮那業)—Kūkai’s subsequent contributions had a much greater immediate impact on how the tradition unfolded.