This chapter is dedicated to the topic of multiphase reactors, characterized by reactions occurring in a liquid phase in which one or more of the reactants, coming from another gaseous or liquid phase, are dissolved. The catalyst promoting the reaction can be a component of the reacting mixture (e.g., gas–liquid reactors), a solid wetted by liquid phase (gas–liquid–solid reactors), or, more seldom, dissolved in another immiscible liquid (liquid–liquid or gas–liquid–liquid reactors). Modeling such systems is challenging because chemical, physical, and fluid dynamic factors are all involved, sometimes giving place to peculiar phenomena. The occurrence of a reaction in the liquid phase containing the catalyst determines the occurrence of gradients at the interfaces, followed by a mass-transfer flow across the interface. The main theories presented in this chapter are two-film theory, surface renewal theory, and penetration theory. Examples presented in this chapter include the oxidation of tetra-hydro-anthraquinone (THEAQH2) and o-cresol alkylation. The MATLAB code associated with these examples is available online.