Thermodynamics provides one of the scientific cornerstones of chemical engineering. This review considers how thermodynamics is and has been used to provide phase equilibria as required for design of standard chemical engineering processes with emphasis on distillation and other conventional separation operations. While this review does not consider "modern" thermodynamics for high-tech applications, attention is given to 50 years of progress in developing excess-Gibbs-energy models and engineering-oriented equations of state; these developments indicate rising use of molecular physics and statistical mechanics whose application for chemical process design is made possible by increasingly powerful computers. As yet, results from molecular simulations have not had a major influence on thermodynamics for conventional chemical engineering; however, it is likely that molecular simulation methods will become increasingly useful, especially when supported by quantum-mechanical calculations for describing intermolecular forces in complex systems. (C) 2004 American Institute of Chemical Engineers.