The aim of this monograph is to summarize the essential features which characterize the behavior of regulatory systems. Firstly we discuss the laws which govern ligand binding in thermodynamic terms. The basic cooperative and allosteric phenomena are des cribed in thermodynamic terms without assuming any particular mo del. Then the molecular models developed by Monod, Wyman and Changeux and by Koshland, Nemethy and Filmer are presented in detail. Special emphasis has been given to the analysis of the Hill coefficient and its meaning both in thermodynamic terms and in terms of the two...