The mechanisms involved in the v(AH) band formation in the H-bonded complexes RA...B and the approximations used to reconstruct the experimental band shapes are discussed. It is emphasized that the high-precision studies of H-bonded systems in the gas phase are important in determining the geometrical and electro-optical parameters of the complexes necessary, in particular, for calculating their properties in condensed phases. The gas phase studies of v(AH) band profiles at room temperature allow one to derive the characteristics of low frequency modes. Analysis of the shape of strong vAH) stretching bands is promising in this respect. Consideration of the H-bonded systems whose spectra are structured, e.g. complexes of HF with HCN and H2O, can provide a larger amount of information. It is shown that for the H2O...HF complex not only the electro-optical parameters, but also the assignment of spectral features can be reliably obtained only from nonempirical calculations, which are quite feasible at present. Th