The question of models of absorption spectra as applied to the problems which arise in investigations of the propagation of infrared radiation in the atmosphere is discussed. The general and asymptotic formulas for the absorption function of individual lines, models of equidistant lines of equal intensity and statistical models are considered successively. Certain new formulas are introduced without mathematical derivation. Inaccuracies and errors in the literature are corrected, and the square-toot law is considered critically. The general conclusions resulting from the investigations into spectral models are analyzed.