The Standard Model could be extended to include sterile (right-handed) neutrinos. The lightest sterile neutrino with a mass in the keV range would be a promising warm Dark Matter candidate. Depending upon the mixing angle, sterile neutrinos have the possibility to decay into active neutrinos by emitting monoenergetic photons in the keV range. Such a line emission could be detected from the Galactic halo or other astrophysical objects like dwarf spheroidals with the X-ray telescopes XMM-Newton, Chandra or Suzaku.