  Active Galactic Nuclei (AGN) are ideal sources for multi-wavelength studies
as their emission can cover almost 20 orders of magnitude in frequency from the
radio to the gamma-ray band. After reviewing their basic properties, I will
assess how well we know the multifrequency spectra of AGN as a class. I will
then briefly illustrate how currently available and forthcoming sky surveys
will help in addressing some of the open questions of AGN studies. Finally, an
analysis of the problem of the missing Type 2 QSO will exemplify the dangers of
monochromatic sky surveys for AGN.
