Internal Shocks and the Blazar Sequence
  We consider the internal shock model as the dissipation mechanism responsible for the emission in blazars. It has been shown that this model is successful in reproducing the observed spectral energy distribution and the variability properties of a powerful blazar like 3C 279. However, the blazar family covers a wide range of spectral characteristics which appear to be correlated and the whole class can be seen as a sequence: the frequency and the intensity of the low energy versus high energy peak intensity increase with decreasing luminosity. We show that the internal shock model can satisfactorily account also for the properties of the low power blazars like BL Lac and Mkn 421 and it is successful in reproducing the blazar sequence. 
D. Guetta, G. Ghisellini, D. Lazzati and A. Celotti