Depending on the (processing) conditions polypropylene can crystallize in different phases. For example in injection molded plates, going from the wall towards the core, we detect; meso, gamma, beta, and alpha as the dominant phase. We have developed an experimental technique (extended dilatometry) to study systematically these multiple phase kinetics as a function of temperature history, pressure, shear rate (i.e. Weissenberg number), and shear time. A model was developed that allocates nuclei (quiescent and flow-induced) to the different crystal phases based on their momentary growth rate, which were measured independently. The final result is a model for multiphase kinetics that can be used in process modeling. We will show the experimental method and results, the structure and basic ideas used in the model and, finally, the model predictions.