Extensive monitoring is now available for a good sample of the brighter quasars and BL Lacs. In some cases, lightcurves are well enough sampled to permit stochastic modelling. The authors present here, a simple model of the lightcurve of the prototype BL Lacertae object, BL Lac in terms of a series of random events, each with rapid rise and exponential decline. Whilst unable to explain very rapid "optically violent" activity, it seems to provide a satisfactory fit to the long term trends in the lightcurve.