The molecular, thermodynamic, and kinetic factors that govern the formation and concentration of the alpha and gamma polymorphs in metallocene-catalyzed isotactic poly(propylenes) have been studied with a set of polymers that have a wide range in molecular weight and defect contents. With these polymers it was possible to investigate the influence of molecular weight on gamma formation at a tired defect concentration, as well as the role of the defect concentration at constant molecular weight, The major experimental techniques used were wide-angle X-ray scattering and differential scanning calorimetry complemented by microscopy. From these studies the role of chain microstructure, the crystallization temperature, and the thermodynamic and kinetic requirement for the formation of the gamma form could be established in more quantitative detail than heretofore. A particular important finding was the fact that at fixed defect concentration the fractional content of the gamma polymorph goes through a maximum with crystallization temperature. The results that were obtained establish a quantitative framework within which the underlying bases that lead to formation of the gamma form, and its unique crystalline structure, are discussed.