Molecules and suspended particulates in the atmosphere scatter and absorb visible and infrared radiation as it passes through the atmosphere. Gaseous absorption bands exist primarily in the infrared part of the spectrum in which the scattering of radiation is less important than in the visible. Nevertheless, if the scattering optical thickness of the medium is greater than about one for uncollimated radiation or greater than about ten for highly collimated radiation, then a photon may be scattered many times as it propagates from source to detector. In addition to multiple scattering, the radiation is scattered predom-inantly into the forward direction for the case of radiation incident upon relatively large particles. These conditions of multiple scattering and a highly anisotropic scattering pattern occur frequently in hazes, fogs, clouds, smoke, and dust. In order to account for the radiant energy at some point within the medium, it is necessary to consider multiple scattering radiation models. A number of detailed mathematical models and computational procedures of varying complexity have been developed in the last twenty years. In this review we consider some of the more practical radiative-transfer models used in the calculation of multiply scattered radiation in optically thick media.