The first two chapters of this book are an update and outgrowth of the monograph Nonequilibrium Phenomena in Polyatomic Gases published by OUP in 1990, and a response to considerable improvements in the experimental determination of the transport properties of dilute gases that have taken place during the past 30 years. The experimental determination has improved sufficiently that it has become necessary to carry out calculations at the level of the second Chapman-Cowling approximation in order to give computed results that lie within the current experimental uncertainties now being reported. Chapter 3 is devoted to realistic interatomic potential energy functions, and begins with a discussion of the need for more accurate representations of these functions. Direct inversion of both microscopic data (spectroscopic transition frequencies and atomic beam scattering) and bulk property data (pressure and acoustic second virial coefficients, transport properties) are discussed in detail. The quantum chemical ab initio determination of binary atomic interaction energies and their analytical representation are discussed, followed by a detailed considerations of the interaction energies between pairs of noble gas atoms. Chapter 4 is concerned with connections between theory and experiment, including a detailed discussion of pure noble gases and their binary mixtures. Chapter 5 focuses on how to obtain the spectroscopic and thermophysical properties of a specific molecular system theoretically step by step, and provides a reference for the specific theoretical calculation work.