After a brief discussion of the history of the introduction of H-functions into radiative transfer theory, the equations for these functions are introduced and explicit expressions given for them. Formulas are also given for calculating the direction and frequency distributions of radiation emerging from a semi-infinite medium with different external and internal sources. An algorithm for calculating Voigt H-functions and related quantities is described in detail. A modern free code has been developed in Fortran language for implementing this algorithm. This code can be used to calculate the profiles of spectral lines with a Voigt absorption line profile with high accuracy. Examples of line profile calculations are shown and a qualitative justification of a hypothesis for the formation of cyclotron features in the spectra of neutron stars is given.