It is generally recognized that Marx's account of human alienation (estrangement) presented in the 1844 Economic and Philosophical Manuscripts is dependent upon his notion of a human species-being. Since that concept is almost totally absent from Marx's later writings, it is not generally acknowledged that this concept continues to play a central function in Marx's writing and, in particular, in his critique of capitalist society. In this paper, I shall try to show the full range of implications that Marx draws from his claim that "Man is a species-being... and free conscious activity constitutes the species -character of man" (Marx, 1974, pp. 327-328 ). In the first sec tion, I will show how Marx's use of the concept of a human species-being is a radical reconceptualization of the philosophic tradition's use of the idea of a human essence. I will then, in the second section, show how this concept functions to ground both Marx's critique of capitalist society as well as his view of socialism as a positive alternative to that society. Finally, in the third section, I will show that this view of the human being leads to a critique of various "ahistorial" theories of human nature present in the philosophic and economic tradition. As a result, we shall see that Marx's conception of the human species-being is a genuine theoretical innovation that functions as the centerpiece of Marx's view of human beings and their society.