The papers in this volume were presented at the Symposium on Steroid Hormone Receptor Systems held October 18-20, 1978, at the Worcester Foundation for Experimental Biology, Shrewsbury, Mass. The meeting was organized to review, discuss, and disseminate new knowledge about the regulation and function of the receptor proteins which mediate estrogen, progestin, glucocorticoid, and androgen action. The symposium brought together leading scientists whose interests span the spectrum of biological organization. On this occasion, Drs. Elwood V. Jensen and Etienne E. Baulieu were honored as recipients of the Tenth Annual Gregory Pincus l1emorial Award for their pioneering studies of steroid hormone receptors. The material covered in this book focuses on the molecular mechanisms which control receptor site availability and function. The topics discussed include recent work on receptor antibodies, antiestrogen action, receptor heterogeneity, nuclear binding and processing of receptor, receptor activation and inactivation mechan isms, interactions between receptor systems, influence of biological rhythms, membrane binding sites, and the evolution of steroid-bind ing proteins. Although studies on steroid receptors have led to the development of new approaches for cancer therapy and contraception, much remains to be discovered in this rapidly expanding field. Hope fully, this book will provide added impetus to the quest for a full understanding of steroid receptor systems by drawing attention to the unresolved questions yet to be answered. Hhile the book is intended primarily for those interested in hormone action, it should be of value to a more general audience including cell, molecular, and developmental biologists.