A framework for modeling constraints on the dynamic behavior of systems obeying the laws of thermodynamics is discussed. These constraints induce dissipativity and we call a system obeying such constraints a process system, passivity theory then shows that low-dimensional macroscopic analysis can be used for control system design. The inputs and outputs converge to their set points and the state vector converges to a stationary passive state provided one exists. Several examples are developed to support the theory and an extension is proposed for systems with equipment constraints.