Rate law can be expressed as a function of the concentrations of the different species. This chapter shows how to use that information to design ideal chemical reactors: reactors with a simplified flow pattern. A reactor is nothing else than a vessel to carry out a reaction. The chapter explains the typical steps involved in the functional design of a chemical reactor. It highlights that the kinetic term of the molar balance can be handled either in terms of concentration or conversion. In a batch reactor (BR), during reaction, no reactants are supplied to the reactor and no products are released. The continuous stirred tank reactor (CSTR) is characterized by a continuous supply of reactants and release of products. This type of reactor can operate at steady or nonsteady state. The chapter concludes with a discussion on the deviations from plug flow and perfect mixing.