Various possible mechanisms of cosmic X-ray emission are discussed. In §1 are summarized experimental results which show local sources in the directions of the constellation of Scorpius and of the Crab nebula as well as a rather strong isotropic intensity in the KeV region. In §2 are given qualitative considerations how difficult the interpretation of such strong emission is. In §3 are discussed several possible mechanisms of X-rays emission: synchrotron radiation, inverse Compton effect, brems-strahlung, innerbremsstrahlung, characteristic X-ray excitation and thermal X-ray emission. None of those mechanisms gives sufficient intensity compared with the observed one, unless some peculiar objects are assumed to exist. Among the possibilities the thermal emission is favoured for the local sources as well as the isotropic intensity. As possible candidates of the former, the neutron stars, the early-type binaries and the peculiar hot regions are suggested, whereas for the isotropic component of the thermal emission in intergalactic space is suggested to be responsible. Non-thermal process can not entirely be ruled out; among them the innerbremsstrahlung associated with knock-on electrons normally provides the most efficient mechanism of X-ray emission.