The most important variables controlling mineral stability are temperature, pressure, and chemical composition (Zoltai and Stout 1992). Polymorphism and its transformation are common and important phenomena in minerals, which can be understood as a special process of mineral transformation under unchanged chemical composition. The relationship between polymorphism and mineral properties is also a typical example of how crystal structure determines physical and chemical properties. Therefore, the polymorphism and similarity of minerals not only have important geological significance but also have important material science significance. This chapter is based on previous work (Wang et al. 1982; Pan 1993, 1994), combined with the research work of the author of this handbook, focusing on the concept of the polymorphism of minerals, the transformation and influencing factors of polymorphism, and the impact of polymorphism on the basic properties of minerals.