  The remarkable Crab Nebula is powered by an energetic pulsar whose
relativistic wind interacts with the inner parts of the Supernova Remnant
SN1054. Despite low-intensity optical and X-ray variations in the inner Nebula,
the Crab has been considered until now substantially stable at X-ray and
gamma-ray energies. This paradigm has been shattered by the AGILE discovery in
September 2010 of a very intense transient gamma-ray flare of nebular origin.
For the first time, the Crab Nebula was "caught in the act" of accelerating
particles up to 10^15 eV within the shortest timescale ever observed in a
cosmic nebula (1 day or less). Emission between 50 MeV and a few GeV was
detected with a quite hard spectrum within a short timescale. Additional
analysis and recent Crab Nebula data lead to identify a total of four major
flaring gamma-ray episodes detected by AGILE and Fermi during the period
mid-2007/mid-2011. These observations challenge emission models of the pulsar
wind interaction and particle acceleration processes. Indeed, the discovery of
fast and efficient gamma-ray transient emission from the Crab leads to
substantially revise current models of particle acceleration.
