Phase equilibrium thermodynamics is one of the most important fundamental sciences for process development in the chemical industry. The outstanding results achieved by thermodynamics in the last decades have led to a profound change in working methods for the development of thermal separation processes. The demands on availability, accuracy, completeness and range of application for thermodynamic data are discussed from the viewpoint of the chemical industry from product development to plant design. Practical examples from the different stages of process development are used to illustrate these points and indicate areas of future research.