The kinetics of supercritical fluid extraction of valuable substances from plants depends on both extract solubility in the solvent (or phase equilibrium between the matrix and the solvent) and mass transfer resistances, particularly the resistance inside vegetable material. Different mathematical models for supercritical fluid extraction have been published in the last decades and it can be therefore difficult to choose the most suitable model for particular extraction. In this chapter, simple criteria based on time constants of mass transfer and characteristic time of equilibrium extraction are presented, two most frequent types of models for supercritical extraction from plants are mentioned with references to the literature, and the factors influencing scale-up of the process are discussed.