  Active Galactic Nuclei (AGN) are known to show significant variability over a
wide frequency range. We review observational results on the variability
characteristics of blazars in the very high energy (VHE) domain, focusing on
recent findings of rapid VHE variability and evidence for an underlying
multiplicative driving process in PKS 2155-304. We explore a physical scenario
where the variability is assumed to arise due to accretion disk fluctuations
transmitted to the jet, and discuss its implications for the central
powerhouse.
