Thermodynamic properties have always played essential roles in the engineering of chemical products and in the processes that manufacture them. Further, contemporary and future chemical technologies depend more than ever on property model formulation and application. This work explores how properties are utilized in process and product engineering, including opportunities and constraints of current property models, the current status of data availability and needs, and the interplay of data and models. Several case studies are given to illustrate underlying concepts, strategies for development, and methods of application to some industrial systems.