Because the issue of Friedrich Nietzsche's influence upon William Butler Yeats has been recently addressed by a number of Yeats scholars, and because their arguments are often marred by misconceptions of Nietzsche's thought, this study undertakes, for the sake of a clearer picture of the aesthetic views of each of these men, a detailed analysis of one concept, that of the mask, for which it has been claimed by certain Yeats critics (Richard Ellmann especially) that a definite and important case of Nietzsche's influence upon Yeats does exist. In Chapter One a review of the literature on Nietzsche in Yeats criticism undertakes to clarify basic misconceptions, which can only serve as obstacles to a proper comprehension of Nietzsche's concept of mask* Chapter Two analyzes this concept as it was developed by Nietzsche over three books; The Gay Science, Thus Spoke Zarathustra, and Beyond Good and Evil, The exegesis makes clear that by this concept Nietzsche meant a form of concealment. Chapter Three then analyzes Yeats's concept of mask, drawing upon writings composed as early as 194, and concludes that by "mask" Yeats meant a form of revelation. The word "mask" is thus seen to denote very different things, as employed by each man in his mature writings, and the question of an influence on this point is then argued to need a fundamental re-interpretation.