This volume attempts to study the origin of co-operatvity in binding systems from the molecular point of view. The molecular approach provides a deeper insight into the mechanism of co-operativity and regulation than the traditional phenomenological approach. This book uses the tools of statistical mechanics to present the molecular theory of co-operativity. Co-operativity is used in a variety of processes, such as the 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.