Polymorphism is one of the most genuine phenomena in high-pressure sci-ence. The existence of different crystalline structures for the same chemical compound and the possibility of keeping some of them as metastable phases at ambient conditions arouse great interest within both the scientific

and the technological realms. We can think of many examples, graphite and diamond carbon allotropes are probably the most common and representative examples. The key to understand metastability in pressure-induced phase transformations lies in the kinetics of the process. This is a difficult topic to deal with, as we will show in this chapter, mainly due to the complexity of describing the time evolution of a transition. Fortunately, mechanistic aspects, the other part inherent to the kinetics of the transition, can be described with approximate models providing valuable information. Thus, we can understand at a microscopic level the energetics, structural and bonding changes associated with the atomic reorganization involved in the transformation.