Polymers are complex molecules where the properties strongly depend on the production process. Moreover, their properties themselves in turn have a strong impact on the process. For example, the molecular weight or the molecular weight distribution may change during the process and so the viscosity, which leads to changes in mixing behaviour, flow properties etc. Such interdependencies are captured by models, which not only take into account mass and energy balances, but also the polymerization kinetics and thermodynamics. By this, these models link process conditions to the molecular structure and to product properties. This facilitates the development of new products and processes and the optimization of existing processes. The process of model-building and the use of such models is demonstrated.