The dramatic increase of viscosity is a fundamental problem when designing a polymerization process. Since a thorough understanding of rheokinetics-the study of the coupled rheological changes and chemical kinetics in the reaction mixture-especially at high degrees of conversion-is still missing, industrial polymerization processes are often designed on basis of trial and error procedures and operate in far-from-optimal conditions. Even at laboratory scale the measurement of the viscosity of systems rapidly changing with time is troublesome and predicting quantitatively the increase of viscosity is not an easy task. The non-Newtonian character of polymeric systems makes rheokinetics an even more challenging field, and more interesting as well. In this review article the difficulties encountered by the research community, the results obtained so far and the problems still to be faced are discussed.