To achieve optimal control of a polymerization reactor means that all the reactor outputs are maintained within specifications. This chapter considers control systems design procedures for multivariable systems and their application to polymerization reactors. The chapter discusses design procedures based on the assumption of linearity, the resulting controllers should be tested on a full nonlinear simulation of the reactor control system prior to implementation. It evaluates the application of pole placement adaptive controllers to a highly nonlinear polymerization process. The chapter presents three applications of multivariable control to free radical polymerization. The first involves the multivariable control of the solution polymerization of methyl metbacrylate in a single Continous Stir Tank Reactor (CSTR). The second application involves the same solution polymerization of methyl methacrylate, but carried out in a semibatch reactor. The last application is to the control of a single CSTR in which emulsion polymeriz.ation of methyl methacrylate is taking place.