  In the framework of the study of extragalactic radio sources, we will focus
on the importance of the spatial resolution at different wavelengths, and of
the combination of observations at different frequency bands. In particular, a
substantial step forward in this field is now provided by the new generation
X-ray telescopes which are able to image radio sources in between 0.1--10 keV
with a spatial resolution comparable with that of the radio telescopes (VLA)
and of the optical telescopes. After a brief description of some basic aspects
of acceleration mechanisms and of the radiative processes at work in the
extragalactic radio sources, we will focus on a number of recent radio, optical
and X-ray observations with arcsec resolution, and discuss the deriving
constraints on the physics of these sources.
