  In the 1990s a comparison of sparse EGRET measurements with single-dish flux
density monitoring from the Metsahovi and UMRAO programs established a temporal
connection between the onset of flaring at radio band and the occurrence of
gamma-ray activity. Correlations between the emergence of new VLBI components
from the core, flares in linearly polarized radio flux, and gamma-ray activity
in bright EGRET-detected blazars supported a picture in which the gamma-ray and
the radio band emission arises in the same shocked region of the jet, with the
high energy emission produced via inverse Compton scattering by the
synchrotron-emitting electrons in the jet. Quantitative tests of this scenario,
however, were hampered by insufficient temporal sampling of the data and the
simple nature of the models adopted. The extensive data from Fermi coupled with
the wealth of well-sampled radio band data from old as well as new programs
such as the F-GAMMA project now permit statistical studies for large numbers of
sources, including weak HBLs, and detailed analyses of individual highly-active
class members. I summarize progress in understanding the origin of the
gamma-ray emission using these new measurements. I focus on three areas:
attempts to isolate the physical site of the high energy emission using time
delay information; investigation of the emission process using the
characteristics of the variability; and quantitative tests of the shock model
picture using high-time-sampled multifrequency linear polarization data, VLBP
imaging, and new models of propagating oblique relativistic shocks
incorporating detailed radiative transfer calculations.
