Chapter 6 starts with a definition of thermophysical properties, followed by detailed descriptions of important terms and equations in diffusion, including Fick’s laws on diffusion; four types of diffusion coefficients (self-diffusion, impurity diffusion, intrinsic diffusion, and interdiffusion); atomic mechanisms of diffusion; diffusion equations in binary, ternary, and multicomponent phases; as well as phases with narrow homogeneity range. Short-circuit diffusion is also briefly mentioned. Subsequently, several computational methods, including first-principles calculations, MD simulation, semi-empirical approaches, and DICTRA software, are presented to calculate or estimate diffusivity and atomic mobilities from which various diffusivities can be computed. Modeling of selected important thermophysical properties, including interfacial energy, viscosity, volume, and thermal conductivity, is briefly introduced. A procedure to establish thermophysical databases is described from a materials design point of view. A case study for simulating age hardening in AA6005 Al alloys is demonstrated mainly using thermophysical properties as input to show their importance for materials design.