The present study is an attempt to formulate a new approach to the modeling of the injection moldings of semicrystalline polymers. A phenomenological approach is employed to develop the qualitative description of crystallization phenomena under nonisothermal flow occurring in various polymer processing operations. The unified crystallization model, which is suitable to describe the multilayer microstructure arising in the injection moldings of semicrystalline polymers, is proposed. The “competing mechanisms” of crystallization in terms of the induction time indices for introducing various microstructure layers are utilized to quantitatively distinguish between the highly oriented skin layer and spherulitic core in the moldings. The application of the equations of continuity, momentum and energy, along with the rheological model, the equation of state and the unified crystallization model, to the injection molding process leads to the modeling of crystallinity and microstructure development in the moldings. The material parameters to be used in the modeling of the crystallinity and microstructure development in the injection moldings of isotactic polypropylenes (i-PP's) with various molecular weights are determined. The melt viscosity as a function of shear rate and temperature is measured and fitted to the modified Cross model. Quiescent crystallization kinetics, including the induction time and the heat release due to crystallization, is characterized using differential scanning calorimetry. The spherulite growth rate of i-PP's under isothermal and nonisothermal quiescent crystallization conditions is measured by optical microscopy. The molecular deformation factor during the induction period of shear-in-duced crystallization is obtained from the extrusion-quenching and extrusion-relaxation-quenching experiments with a slit die. The variation of the induction time with shear rate obtained by Lagasse and Maxwell from the shearing-crystallization experiments is used to determine the shear enhancement coefficient of crystallization.