This chapter presents a generalized P-T-t diagram of the evolution of metamorphic complexes of different geodynamic nature that are characteristic of different types of metamorphism. This diagram was built using the most recent natural observations, which are characterized by the simultaneous presence of prograde and retrograde segments of a P-T path. This chapter discusses some of the ambiguous interpretations of P-T-t paths in areas with simultaneous manifestation of different metamorphic processes. Quantitative models for the analysis of different types of interaction of rocks undergoing metamorphism are exemplified using the following processes involved in the formation of distinctly expressed zoned reaction textures (coronites, kelyphites, symplectites, nodules, and segregations) and mineral transformations in texturally homogeneous rocks between matrix minerals and porphyroblasts. It was shown that mass transfer during metamorphic reactions occurs with the preservation of the material balance within very small local volumes of the rock, which increase from a few hundredths of a mm3 to a few cm3, depending on the duration of metamorphism, P-T parameters, strain intensity and a degree of fluid saturation of rocks. We also consider different kinetic parameters of diffusion-controlled metamorphic reactions, such as mineral reaction mechanisms and rates.