Physical models of the theory of infrared radiation transfer in molecular gases are reviewed and their usefulness in the solution of particular problems is demonstrated. Theoretical predictions are compared with experimental data. Particular attention is devoted to analytic methods capable of providing (1) clear descriptions of the physics of the phenomenon, (2) a better understanding of the dependence of the final result on the parameters of the problem, and (3) a reasonably reliable final result that is valid over a broad range of variation of the parameters of the gaseous medium. The radiation transfer process is examined both for equilibrium and nonequilibrium gaseous media. The final section deals with the analysis of certain processes occurring in the Earth's atmosphere.