The present mograph on Plato's Sophist developed from series of lectures given over a number of years to hours 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 tice after my work was substantially completed (such as KamIah's and Sayre's) have t been referred to in my discussion. As few philosophy students wadays are familiar with Greek I have (except in a few foottes) 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 table 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.