  We review the continuum emission of Active Galactic Nuclei (AGN) over the
entire electromagnetic spectrum. After a brief historical introduction, we
describe the main spectral properties of unobscured AGN, discussing the
selection biases which prevent us from having a complete view of the AGN
population in the universe, and trying to build an updated spectral energy
distribution of optically selected quasars. In the second part of the review,
we describe the spectral properties of obscured AGN. Finally, we discuss the
main observational methods in the different wavelength bands for disentangling
AGN and stellar emission, and the ability of these methods to find new (mainly
obscured) AGN, a significant fraction of which are probably still missing in
current surveys.
