The purpose of this book is to serve as a primer for the study of cell surface receptors. The simplified discussion of methods and their underlying prin- ciples is intended to remove the usual intimidation caused by the specialized vocabulary or sophisticated mathematics that characterize many of the primary papers in this field. In this way, it is hoped that the basic concepts can be emphasized. This book is meant to be a starting point: a textbook as well as a manual to which the investigator can return for a refresher course, when needed. I feit compelled to write this book for several reasons. The primary philosophical reason was to provide, in one volume, an overall perspective on the study ofcell surface receptors that describes their characterization from pharmacological studies in a whole organ or tissue bath (chapter 2) to studies of radioligand binding in isolated membrane and detergent-solubilized pre- parations (chapters 3-5) and, back again, to studies in intact cells or tissues using morphological and biochemical techniques (chapter 6). It is my opinion that the ultimate understanding of the molecular basis for hormone or drug- receptor interactions, and the physiological consequences of these inter- actions, will require an assessment of cell surface receptor function at all of these experimental levels. In many cases an investigator may not choose to of investigation considered here.