The present monograph on Plato's Sophist developed from series of lectures given over a number of years to honours and graduate phi losophy classes in the University of Waterloo. It is hoped that it will prove a useful guide to anyone trying to come to grips with, and gain a perspective of Plato's mature thought. At the same time my study is addressed to the specialist, and I have considered at the appropriate places a good deal of the scholarly literature that has appeared during the last thirty years. In this connection I regret that some of the pub lications which came to my notice after my work was substantially completed (such as KamIah's and Sayre's) have not been referred to in my discussion. As few philosophy students nowadays are familiar with Greek I have (except in a few footnotes) translated as well as transliterated all Greek terms. Citations from Plato's text follow Cornford's admirable trans lation as closely as possible, though the reader will find some significant deviations. The most notable of these concerns the key word on which I have rendered throughout as being, thus avoiding Cornford's existence and reality which tend to prejudge the issues which the dialogue raises.