A general introduction to and bibliography for transport phenomena in gases is provided. Methods for obtaining density expansions of transport coefficients from time-correlation functions in a moderately dense gas with short-range repulsive intermolecular forces are considered. A unified treatment of the two methods appearing in the literature (the $t$ method due to Cohen, Dorfman, and Ernst and the $\epsilon$ method due to Zwanzig) is given. Both of these methods lead to integral equations from which the first two terms in the density expansion of transport coefficients can be computed. However, because of many-body effects in the gas, both methods diverge when used to compute terms beyond the first two in these density expansions. Because of this divergence, it is necessary to prove that the $t$ and $\epsilon$ methods give the same results for the first two terms in the density expansion of transport coefficients. The required proof is provided, and we conclude that either the $t$ or $\epsilon$ method can be used to compute the first two terms in the density expansion of transport coefficients provided one assumes that the remaining (divergent) terms, which are neglected, do not contribute to the first two terms.

DOI:https://doi.org/10.1103/RevModPhys.41.296