With this chapter, a new part of the book begins, dedicated to chemical reactors. They should be seen as strictly interconnected to the unit operations (with related transport phenomena) we have seen so far, since it is plausible that a reactor includes unit operations (e.g., various separations, heat exchanges) both upstream (for the preparation of the reacting feed) and downstream (for obtaining the desired products in their final form). This chapter introduces the general concepts related to reaction rate and degree of conversion, and then it presents the three ideal chemical reactors (discontinuous with perfect mixing, continuous with perfect axial segregation, continuous with perfect mixing), each with its fluid dynamic aspects and design equation. Reactors are then accompanied by considerations related to productivity in the desired species.