After the examination of the historical evolution of the industrial production of polymers, different polymer classifications are described. Then, the polymers’ average molecular weight and molecular weight distribution are defined together with the description of the techniques commonly employed for their experimental determination. The different mechanisms of polymerization and the kinetics of polymerization are described as a base for the polymerization reactor modeling. For this purpose, polycondensations, polyadditions, radical polymerizations, and stereospecific polymerizations are considered in detail for both the mechanism and kinetics. A paragraph devoted to polymerization reactor modeling concludes the chapter. Different solved examples and exercises are developed such as the following: kinetics of polycondensation, conversion from molecular weight distribution, kinetics of polyethoxylation, the method of moments applied to kinetics of polymerization, stepwise polymerization in different reactors, batch, plug flow, CSTR, and styrene radical polymerization in a batch reactor.