  I review X-ray observations of classical and recurrent novae in outburst.
Significant X-ray flux is emitted by the nova shell, with a peak luminosity up
to 10**35 erg/cm**2/s in the 0.2-10 keV range.In recurrent nova systems, or in
novae hosting a red giant, the source of X-rays may be previous circumstellar
matter shocked by the nova wind. However, for most classical novae, X-rays
originate inside the nebula ejected in the outburst. The data indicate a very
high fraction of shocked material, and a non-smooth, varying wind outflow. A
nebular emission line spectrum is also observed at late phases. In about half
of the observed novae, the central white dwarf appears as a very luminous
supersoft X-ray source for 1 to 9 years after the outburst. It is the best type
of object to study the characteristics of shell hydrogen burning on white
dwarfs in single degenrate systems. The available statistics indicate that the
duration of the supersoft X-ray phase is peaked around 2 years. Recently,
``template grating spectra'' with high S/N have been obtained for V4743 Sgr.
  The X-ray light curve of this nova reveals a rich and complex power spectrum,
with signatures of non-radial g-mode oscillations of the white dwarf. The
oscillations and the spectra allow to determine the properties of the shell
hydrogen burning white dwarf.
