During processing of a semi-crystalline resin, a molten polymer flows at temperatures lower than the thermodynamic melting point. In most of cases the crystallization process takes place while the polymer is still flowing and this induces a dramatic change of rheological properties and sometimes an anticipated and unpredicted solidification. The description of the effects of crystallinity on flow is still challenging, mainly because of the difficulties in measuring simultaneously rheological properties and crystallinity evolution. Furthermore, the problem is complicated by the fact that crystallization rate increases by effect of flow, and thus the test itself modifies the material behavior. This is the other side of the phenomenon, which by itself is of great practical interest and is equally challenging: the so-called “flow-induced crystallization”. Investigations aimed at the description of the mutual effects of flow and crystallization have been carried out over the years at the University of Salerno by adopting the same polypropylene. These experiments started from the basic observation of the changes in complex viscosity during isothermal crystallizations, passing through the measurements of nucleation and growth rates during continuous shear and oscillatory flow, going to the measurements of rheological properties (including shear and complex viscosity) at different degrees of crystallinity, up to the latest experiments which have been conducted in a transparent rheometer to directly follow the morphology evolution while measuring the rheological properties. The results of these experiments are here organically summarized and a series of conclusions are drawn which allow to elucidate the correlations between flow and crystallinity. Keywords: Crystallization, Nucleation, Growth, Solidification, Polypropylene