Characterization of the binding of a hormone to its cognate receptor is a cornerstone of many studies in molecular and cellular endocrinology since this event represents the beginning of a specific cellular response, generally from a highly regulated extracellular messenger. The premise of hormone−receptor interaction follows from the law of mass action describing a reversible second-order reaction, hormone plus receptor, to give a non-covalently associated hormone-receptor complex. From this basic principle, a host of useful experimental parameters are available to the interested investigator. This chapter is focused on development of the experimental and mathematical underpinning of hormone−receptor interaction, with emphasis on a gonadotropin, chorionic gonadotropin (or luteinizing hormone), binding to the luteinizing hormone receptor, a member of the G protein-coupled receptor family. The general concepts and approaches developed herein are, however, valid to most interacting systems.