  Relativistic jets in AGN in general, and in blazars in particular, are the
most energetic and among the most powerful astrophysical objects known so far.
Their relativistic nature provides them with the ability to emit profusely at
all spectral ranges from radio wavelengths to gamma-rays, as well as to vary
extremely at 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
other facilities at most of the available spectral ranges (including millimeter
interferometric imaging and polarization measurements wherever possible) are
starting to shed light for the case of blazars. After a short review of the
status of the problem, we summarize two of our latest results -obtained from
the comprehensive monitoring data compiled by the Boston University Blazar
monitoring program - that locate the GeV flaring emission of the BL Lac objects
AO 0235+164 and OJ287 within the jets of these blazars, at >12 parsecs from the
central AGN engine.
