X-ray lasers (XRLs) are presently the brightest source in the wavelength range comprised between a few nanometers and a few tens of nanometers; they are brighter by orders of magnitude than any other source. The XRL active medium is a hot and dense plasma generally produced by a high power infrared laser, although capillary discharge has shown to be able to produce amplifying plasmas. Population inversions are due to electron-ion collisions. Nevertheless, other pumping schemes, particularly in rapidly cooling plasmas, are investigated. Concurrently high-harmonic generation gives rise to short wavelength, coherent emissions.