  As a result of recent observations with ROSAT and ASCA the number of
rotation-powered pulsars seen at X-ray energies has increased substantially.
  In this paper we review the phenomenology of the observed X-ray emission
properties. At present 27 pulsars are detected, representing a wide range of
ages (10^3 - 7 x 10^9 yrs), magnetic field strength (10^8 - 10^13 G) and spin
periods (1.6 - 530 ms). Despite these dispersions in parameters all pulsars
show an X-ray luminosity closely correlated with the rotational energy loss.
This suggests that most of the observed X-rays are produced by magnetospheric
emission originating from the co-rotating magnetosphere.
  Only for three middle aged pulsars (PSR 0656+14, Geminga and PSR 1055-52) and
probably for the Vela-pulsar an additional thermal component is detected which
can be attributed to thermal emission from the neutron stellar surface.
