The aim of this thesis was to evaluate the γ-ray survival probability after taking into account the scattering on the background photon field. For that purpose we wrote the code that calculates the survival probability for γ rays with a source inside and outside of the Milky Way. In this thesis we presented models of the background photon field and numerical methods we used in the code. At the end we presented dependence of survival probability on γ-ray energies for three different sources: the galactic centre, the Crab Nebula and Markarian 421. To generate these results, the only user input needed in the code are coordinates of γ-ray source in galactic coordinate system. We have seen that for the γ-ray sources inside the galaxy, most of absorption is a consequence of their scattering with the dust radiation and the cosmic microwave background radiation. For extragalactic sources greatest effect on γ-ray absorption is due to the cosmic microwave background radiation and the extragalactic background radiation. For further improvements of the code it is needed to estimate uncertainties of result and improve models used in describing the background photon field.