Abstract　 　 
Gamma-ray emitting active galactic nuclei (AGN) are characterized by variability on a wide range of timescales across nearly the entire electromagnetic spectrum. This is an indication of the dominant role that the jet plays in radiation production. The variability provides a valuable tool to study, in a relatively model-independent way, the emission processes and geometry as well as the energetics of the jet in AGN, not so different from the study of gamma ray bursts. In this talk, I will show recent discoveries of rapid gamma-ray flaring of blazars and discuss the challenges the phenomenon presents to our theoretical understanding of very-high-energy gamma ray production in the jet of AGN.