This is the story of steroid-protein interactions as one investigator sees it. Following the general concept of this monograph series, it emphasizes the results and interpretations obtained in the author's laboratory, and is therefore a subjective account. Recognizing, how- ever, that the discussion of the subject would be incomplete if the material were limited to one scientist's work, the essential achieve- ments of other investigators have been incorporated. An effort has thus been made to give a balanced presentation and to enable the reader to see in perspective the varied facets of the interactions between steroids and proteins. Since this is the first comprehensive treatment of the topic, it seems appropriate to go to the roots, and try to find out how it all started. The first chapters, therefore, take the reader to the laboratories of those who very early conceived the significance of the attachment of dyes, drugs, and other conspicuous molecules to those colloids called proteins. The discovery of the steroid hormones set the stage for meaningful investigation of their interaction with proteins of various origins and functions - a process which is continuing today with increasing vigor.