First recognized in 1822, polymorphism of crystals is now a widely recognized and observed phenomenon, with both fundamental and commercial ramifications in disciplines and industries that study and utilize solid forms of matter. The purpose of this edition is to summarize and to bring up to date the current knowledge and understanding of polymorphism in molecular crystals, and to concentrate it in one source. The information has been gleaned from a wide variety (~2500) of sources in the open literature; however, because of the increasing commercial importance of the phenomenon, a significant portion of the information is less accessible, we have attempted to include both the information from those sources as well with full details of their citations. An introductory chapter on fundamental concepts, definitions, and historical development is followed by a presentation of the physical and structural bases for crystallization and polymorphism. The exploration of the crystal form landscape is described in detail, including polymorph screens, concomitant polymorphs, and disappearing polymorphs. A survey of analytical methods for studying and characterizing polymorphs is followed by a discussion of rapidly developing computational methods for studying and attempting to predict polymorphic behavior. A chapter with many examples of the utilization of polymorphic systems to investigate structure–property relationships in solids precedes three individual chapters on the role and importance of polymorphism in pharmaceuticals, high energy materials, and pigments. The book closes with a chapter on the role of polymorphism in establishing and protecting intellectual property connected with polymorphs through the patent system.