Plantwide control is a holistic approach about integrating process design with process control at the level of the whole plant. The objective is to ensure a stable and flexible operation when the plant is affected by various disturbances, including major changes in the production rate and in the quality of raw materials. A key topic is the control of species inventory, which can be achieved either by feedback or by relying on self-regulation. The applicability of these alternatives is discussed by means of case studies of increasing complexity, for which detailed design, control and dynamic simulation results are provided. The non-linear behaviour of Reactor-Separation-Recycle systems is also discussed. The subject gives the opportunity of reviewing basic concepts of process control.