The two-port control system is introduced to develop the design framework for multivariable predictive controllers. It constitutes a general form for all two-degrees-of-freedom control systems and is particularly suited for the optimum resolution of controller design trade-offs, such as performance to command following versus disturbance rejection, and performance in the presence of various disturbance inputs. It allows the handling of systems with non-minimum phase characteristics or inherently unstable open-loop dynamics. Finally, it offers the necessary and sufficient conditions for the design of effective adaptive control strategies.