  Blazars are high-energy engines providing us natural laboratories to study
particle acceleration, relativistic plasma processes, magnetic field dynamics,
black hole physics. Key informations are provided by observations at
high-energy (in particular by Fermi/LAT) and very-high energy (by Cherenkov
telescopes). I give a short account of the current status of the field, with
particular emphasis on the theoretical challenges connected to the observed
ultra-fast variability events and to the emission of flat spectrum radio
quasars in the very high energy band.
