This chapter briefly explains the development of the subjective approach to value theory. We do not seek to provide a complete history of this theory, but rather to provide a contrast to the objective theory of value about which this book is primarily concerned. The subjective theory of value differs significantly from the objective theory just discussed and its widespread acceptance by 1900 altered the face of economics. We will begin with a brief account of how the subjective theory of value came to dominate the economic way of thinking. The chapter ends with an attempt to isolate the chief characteristics of this theory.