In this thesis I shall summarize and critically examine the central features of the theories of values of four contemporary moral philosophers: A.J. Ayer, C.L. Stevenson, R.M. Hare, and P.H. Nowell - Smith. I shall first look back, however, to the theory of moral philosophy of the most influential 'forefather' of this group, David Hume. Hume's theory stands as a challenge to moral philosophers who would assume that moral judgments are primarily, in some sense, acts of 'reason'. Although our four contemporaries follow Hume in this, his challenge, in the form I shall indicate shortly, will provide the main theme for this thesis. There are of course other important philosophers who have set forth theories similar to those I have chosen, and the names of Carnap, Schlick, Barnes, Findlay, and Urmson immediately come to mind. A larger effort than this would be remiss if it did not give them special attention. The attempt to classify the philosophers in this tradition is reminiscent of Wittgenstein's example of the problem of defining the word "games", although in the case of our philosophers there is no single term which seems appropropriate to separate them from some other types of philosophers. Like the activities we call "games" our philosophers have similarities and dissimilarities, and yet there are enough similarities to suggest that they have 'family resemblances'. They do not suffer from the lack of an appropriate 'family name', so long as one keeps in mind their 'family resemblances'.