  I describe the different classes of Active Galactic Nuclei (AGN) and the
basic tenets of unified schemes. I then review the properties of the
extragalactic sources detected in the GeV and TeV bands, showing that the vast
majority of them belong to the very rare blazar class. I further discuss the
kind of AGN GLAST is likely to detect, making some predictions going from the
obvious to the likely, all the way to the less probable.
