The following chapters comprise invited contributions from eminent scientists who are internationally recognized authorities on the subject. The chapters have been written to provide the reader with adequate background, necessary experimental details, and discussion that is easy to comprehend. The book has been organized into four different sections. The introductory chapter is meant to briefly summarize the contributions of other authors and discuss phosphorylation of receptor systems not covered in the book in detail. The chapters in the second section, on protein kinases and phosphatases, are quite fundamental to the process of phosphorylation and will aid the reader in appreciating the observations and discussions reported in other chapters. The third and fourth sections present discussions on phosphorylation of various receptor systems which are involved in mediating actions of peptide, amine, and steroid hormones. It is hoped that this book will serve as a valuable resource and will be useful to all workers and students in the area of receptor-ligand interactions.