  We confront theoretical models for the rotational, magnetic, and thermal
evolution of an ultra-magnetized neutron star, or magnetar, with available data
on the Anomalous X-ray Pulsars (AXPs). We argue that, if the AXPs are
interpreted as magnetars, their clustering of spin periods between 6 and 12
seconds, observed at present in this class of objects, their period
derivatives, their thermal X-ray luminosities, and the association of two of
them with young supernova remnants, can only be understood globally if the
magnetic field in magnetars decays significantly on a time scale of the order
of 10^4 years.
