One of the Six Schools of Nara Buddhism that flourished during the earliest period of the religion's transplantation to Japan. ‘Ritsu’ means ‘Vinaya’, or the set of regulations and procedures that governs the lives of monks andnuns and the institutions in which they reside, and the Ritsu school saw itself as the Japanese successors of the Chinese Lü school, which specialized in Vinaya study. Because of its specialized knowledge, the school also administered ordinations for all monastics in Japan until Saichō (767–822) received permission from the court in the year of his death to conduct ordinations for his nascent Tendai school without Ritsu control.
The early history of the school is unclear. When Buddhism first arrived in Japan in the 6th century, the procedures and conditions for valid ordinations were unknown, and Japanese who wished to seek ordination either ordained themselves unofficially or went abroad. Within the first few decades, cleric-scholars studied imported Vinaya texts and commentaries, and began to conduct ordinations at several temples, while private and foreign ordinations continued. Two hundred years later, in the 8th century, there still existed sufficient doubt as to the validity of these procedures that the emperor sent an embassy to China in 732 to find a Vinaya master willing to travel to Japan to train students in Vinaya and oversee ordinations. After discovering that, according to Chinese Buddhist standards, their own ordinations were invalid, the Japanese clerics stayed in China to receive the precepts in an orthodox ceremony, and persuaded several Chinese clergy, notably Tao-hsüan (Jap., Dōsen, 702–60) and Chien-chen (Jap., Ganjin, 688–763), to make the journey. Chien-chen, a widely renowned Vinaya master, made many attempts to cross the straits to Japan and finally arrived in 754. The arrival of the Chinese masters caused some tension at first, since the Ritsu school had already been in place for over a century and had settled into its own pattern. After some initial successes, Chien-chen retired from public life and settled in his own temple, where he could train students and conduct ordinations in his own way. Nevertheless, the fresh infusion of scholarship and orthodox practice gave the Japanese clergy the assurance that their ordinations were valid and acceptable within the Buddhist world at large. See also Lü-tsung.