An Overview of Blazar Variability
  Blazars are characterized by rapid variability at virtually all wavelengths from radio through TeV gamma-rays. The challenge since their discovery has been to understand the origin of their luminous, apparently nonthermal, nuclear emission. Considerable progress has been made in recent years thanks to a handful of multiwavelength monitoring campaigns with high enough temporal sampling to resolve the most rapid variations. The best data for a few objects have shown a variety of behaviors, for the most part commensurate with synchrotron and Compton-scattered emission from a relativistic jet, though better data for more blazars are still clearly needed. In particular, the origin of the seed photons that are upscattered to gamma-ray energies remains unclear. The latest multiwavelength light curves for the BL Lac object PKS2155-304 appear to rule out synchrotron emission from a homogeneous source. 
C. Megan Urry (Space Telescope Science Institute)