  The synchrotron emission observed in plerions depends on the characteristics
of the magnetic fields and relativistic electrons in these supernova remnants.
Therefore an analysis of the spectral and spatial properties of this emission,
combined with models for the evolution and structure of plerions, would allow
one to investigate the evolution of the synchrotron nebula, the structure of
the magnetic field, the distribution of the relativistic electrons, as well as
to put constraints on the history of the energy input from the associated
neutron star. The identification of a new class of plerionic remnants, with
spectral properties different from the Crab Nebula, has been proposed. The
spectra of these objects typically show a sharp spectral break at very low
frequencies (below 50 GHz), with a steep spectrum right beyond the break. In
order to model the properties of their emission, a non-standard evolution of
the pulsar output seems to be required. X-ray observations of the synchrotron
emission from the Crab Nebula are shown. They are compared with previous data,
and their implications on the structure of the Crab are discussed. Recent
millimetric data of this object are also presented. A spatially resolved
analysis, based on radio, millimetric and X-ray data, will be carried on also
for other plerions.
