The lecture deals with theoretical principles of macromolecules formation by both polycondensation and polymerization reactions with attention to living polymerizations. Monomers - compounds able to form polymers - are discussed from different aspects. Mechanisms of raising active centers and possibilities of their transformation are summarized. The lecture compares aspects of mechanisms and kinetics of radical, anionic, cationic and coordination polymerizations. It deals with elementary initiation, propagation, transfer and termination reactions, the living polymerizations being accentuated. It describes reaction mechanisms of both standard and special copolymers formation and explains problematics of polycondensation and polyaddition reactions.