In this chapter the current state of the kinetic theory of dilute gases is reviewed. It is shown that the advent of highly accurate ab initio interaction potentials made it possible to calculate the transport properties of both monatomic and simple polyatomic gases over a wide range of temperatures with an accuracy that often exceeds that of the best experimental data. The results obtained for helium, argon, helium–krypton mixtures, hydrogen, methane and water vapour are presented in detail and the accuracy of the calculations is analysed by comparison with experimental data.