Rheology describes the flow and deformation of materials. Plastics in particular are characterized by their special flow behavior. The viscosity of plastics is not only dependent on temperature and pressure, but also on the flow velocity. This flow behavior, known as structural viscosity, is clearly described in this textbook. In addition, due to their molecular chain structure, plastics have visco-elastic properties that affect the flow processes. Understanding of all of these aspects is highly important for developing and maintaining effective, reliable, efficient, and economical plastics processing procedures to produce high-quality products. In this book, firstly different measurement methods that are suitable for measuring these properties are presented and their measurement results are also discussed. The laws that describe the special flow behavior of plastics are then explained. After deriving the flow equations, these are then used to calculate flow processes for the extrusion and injection molding processing methods. In addition to the general basics, there are practical calculation examples for each chapter. The reader thus has the opportunity to study the basics and at the same time apply the knowledge gained. Many full-color explanatory figures are provided, as are QR codes link to videos that explain the content even more clearly. It is therefore ideal for a wide range of people working with plastics in industry, such as injection molders, mold makers, and simulation experts.