 Kūkai called his form of Buddhism, Shingon, which means “the word of truth.” This is the Japanese pronunciation of the Chinese chen-yen (zhenyan) and the Sanskrit mantra. The form of Buddhism that he learned in China was an importation from India of Mantrayāna (Mantra vehicle {or: “school”}) Buddhism, more often known as Vajrayāna (Diamond {or: “Lightning Fast”} vehicle). The word mantra signifies a secret teaching inexpressible in ordinary words or linguistic formulations. This form of Buddhism, whose origin dates back to the mid-seventh century India, is more popularly known today in the West as “Tantric Buddhism,” especially in its Tibetan incarnation. It is characterized by its emphasis upon rituals, involving the use of mantras, mandalas, and other means in order to control both body and mind to experience the “ultimate.” Two major texts, associated with this form of Buddhism, made their way into China from India: Mahāvairocana Sūtra (Jpn: Dainichi-kyō) from the mid-seventh century and Vajraśekhara Sūtra (Jpn: Kongōchō-gyō) from the later-seventh century. Mantrayāna made its way east to China during the T’ang dynasty (713–765) around the beginning of the eighth century through the translating and propagating efforts of the Indian priests Śubhākarasimha (Jpn: Zenmui; 637–735) who brought the Mahāvairocana Sūtra to China; Vajrabodhi (Jpn: Kongōchi; 671–741), the sixth patriarch of Mantrayāna, who brought the Vajraśekhara Sūtra to China; and Amoghavajra (Jpn: Fukū, an abbreviation for Fukūkongō; 705–74), Vajrabodhi’s disciple, who won the support of successive emperors during the T’ang period; and the Chinese priest, I-hsing (Yixing; Jpn: Ichigyō; 683–727) who learned the teachings of Mahāvairocana Sūtra from Śubhākarasimha and the teachings of Vajraśekhara Sūtra from Varjabodhi. Kūkai’s own teacher Hui-kuo had also received initiation under both lineages of these two major sūtras, and is credited with gathering together their teachings into one system. In the meantime during the eighth century, another branch of Mantrayāna made it north from India to Tibet. But Chen-yen Buddhism in China did not survive the severe persecution of Buddhism in general in 845. Kūkai however had by then already succeeded in transmitting to Japan the esoteric doctrines he had studied in China. For much of the doctrines and practices, which he had imported, became influential amongst the official Nara monasteries. But his form of Buddhism was not just a product of his inheritance and appropriation of the Buddhism he encountered in China. As it involved developments and contributions uniquely his own, we may regard him not only as the transmitter but as the founder of the Japanized version of Mantrayāna known as Shingon.