This is the first book that attempts to study the origin of cooperatvity in binding systems from the molecular point of view. The molecular approach provides a deeper insight into the mechanism of cooperativity and regulation, than the traditional phenomenological approach. This book uses the tools of statistical mechanics to present the molecular theory of cooperativity. Cooperativity is used in a variety of processes-such as loading and unloading of oxygen at relatively small pressure differences; maintaining an almost constant concentration of various compounds in living cells; and switching on and off the reading of genetic information. This book may be used as a textbook by graduate students in Chemistry, Biochemistry and Biophysics, and will also be of interest to researchers in theoretical biochemistry.