Locating the gamma-ray emission region in AGN from multi-messenger observations

Relativistic jets in AGN, in general, and in blazars, in particular, are among the most energetic and powerful astrophysical phenomena known so far. Their relativistic nature provides them with the ability to emit profusely at all spectral ranges from radio wavelengths to gamma-rays. They display extreme variability at all time scales (from hours to years). Since the birth of gamma-ray astronomy, locating the origin of gamma-ray emission has been a fundamental problem for the knowledge of the emission processes involved. Deep and densely time sampled monitoring programs with the Fermi Gamma-ray Space Telescope and several other facilities throughout most of the available spectral ranges (including polarization measurements where possible) are starting to shed light onto the mechanisms at work in blazars. After a short review of the state-of-the-art, some of the latest results will be presented, that locate the GeV emission in the jets of some blazars at >10 parsec from the central AGN engine. The implications for the gamma ray emission mechanisms will be discussed.