The monograph is intended to systematize data on active sites and their kinetic and stereospecific heterogeneity (polysite phenomenon) in olefin and diene monomer ion, ion-coordination and complex radical polymerization processes. Questions of molecular-mass distribution of polymers obtained by ion and ion-coordination polymerization methods are discussed together with composition and stereo regularity distribution of olefin copolymers. Data on the influence of transition and non-transition metal origin and their ligand ambience, conditions of catalytic system preparation and polymerization are shown. Various assessment methods of polymerization active site distribution by kinetic activity and stereospecific action are considered. In particular, an assessment method for kinetic activity distribution at ion-coordination diene polymerization is described, mathematical simulation parameters for polymerization processes are shown, taking into account the polysite structure of catalytic systems. Of special attention are geometrical and electron structure of active sites in the ion-coordination polymerization by the quantum chemistry method, interrelation between reactivity of some types of active sites and stereospecificity of their action in diene polymerization. The importance of ratio between specific time elementary act duration at the diene injection, coordinated on an active site, and time of propagating macrochain end unit existence in the s-state at polydiene microstructure formation is indicated. Questions of the polysite mechanism occurrence in complex radical polymerization processes are discussed. A possibility for polymerization to proceed in both free-radical and complex related chain propagation sites is shown.