9For more than seven centuries the Mumonkan has been used in Zen monasteries to train monks and to encourage the religious development of lay Buddhists. It contains forty- eight koans, or spiritual riddles, that must be explored during the course of Zen training. Shibayama Zenkei (1894-1974), an influential Japanese Zen teacher and calligrapher who traveled and lectured throughout the United States in the 60s and 70s, offers his own commentary alongside the classic text. The Gateless Barrier remains an essential text for all serious students of Buddhism.