Polyreactions exhibit a number of peculiarities, the reaction engineering implications of which are discussed in this chapter. First, calculation methods for product distributions are presented, which are essential for the properties of the resulting polymers. Subsequently, the rheology of reaction mixtures is discussed qualitatively, which in polyreactions is often characterized by a significant increase in viscosity with conversion. Criteria for reactor selection, non-ideal flow behavior, as well as selectivity effects and heat removal in polymerization reactors are then discussed. Finally, important polymerization processes and the reactor types used there are briefly presented.