external photons that originate from the accretion disk [@DermerSchlickeiser], from nearby massive stars, or are reflected into the jet by surrounding material [@SikoraBegelmanRees]. In hadronic models, interactions of a highly relativistic jet outflow with ambient matter [@DarLaor; @Bednarek1993], proton-induced cascades [@PIC], synchrotron radiation off protons (proton synchrotron blazar; Aharonian 2000; Mücke & Protheroe 2001), or curvature radiation, are responsible for the high energy photons. The prime scientific interest in BL Lac objects is twofold: (1) to understand the VHE $\gamma$-ray production mechanisms, assumed to be linked to the massive black hole in the center of the AGN, and (2) to use the VHE $\gamma$-rays as a probe of the extragalactic background light (EBL) spectrum between about $0.3$ to $30~\mu\rm m$ wavelength. In the past, most of the VHE $\gamma$-ray emitting AGN were discovered during phases of high activity, biasing our current observational record towards high emission states. Although these sources also show variability in the X-ray, optical, and radio domain, the VHE variability is observed to often be the most intense and violent one. While fast variability on the timescale of 10 minutes has been observed for Mkn 501 in the X-ray domain [@XueCui], flux doubling times well below 5 