  The Anomalous X-ray Pulsars (AXPs) are a small class of pulsars with spin
periods in the 6-12 s range, very soft X-ray spectra, secular spin down on time
scales of ~10^3-10^5 years, and lack of bright optical counterparts. Two,
possibly three, of them are close to the centre of shell-like supernova
remnants. AXPs are one of the most enigmatic classes of galactic high-energy
sources. Isolated neutron stars powered by the loss of rotational energy can be
excluded on energetic grounds. The models based on neutron stars involve either
accretion (perhaps from a fossil disk around an isolated neutron star) or the
decay of a very strong magnetic field (10^14-10^15 G). We review the models and
the observational properties of AXPs, including recent XMM-Newton and Chandra
observations. We also present some unpublished ASCA data.
