"The Gateless Barrier" is generally acknowledged to be the fundamental koan collection in the literature of Zen. Gathered together by Wu-men (Mumon), a thirteenth-century master of the Lin-chi (Rinzai) school, it is composed of forty-eight koans, or cases, each accompanied by a brief comment and poem by Wu-men.
Robert Aitken, one of the premier American Zen masters, has translated Wu-men's text, supplementing the original with his own commentary -- the first such commentary by a Western master -- making the profound truths of Zen Buddhism accessible to serious contemporary students and relevant to current social concerns.