In this paper we use the first and second laws of thermodynamics to motivate a theory for nonlinear process control. Our main tenets are : positive entropy production, boundedness of entropy in terms of energy, concavity of the entropy density and Helmholtz free energy as a storage function. These give the process system a causal input-output description, zero state detectability and stabilizability. To make the theory apply to practical systems we follow ideas from classical irreversible thermodynamics and extend the concept of entropy of the non-equilibrium by assuming local equilibrium.