The Crab Nebula, with its powerful pulsar, is the remnant of a supernova explosion observed in 1054. The Nebula emission for energies ranging from radio to gamma rays (up to 100 MeV) is dominated by synchrotron radiation of electrons accelerated by the pulsar, and shows a cutoff around 100-200 MeV. Because of its very stable emission, the Crab Nebula was considered a calibrator for High Energy Astrophysics telescopes. A big surprise was the discovery by the AGILE satellite of a gamma-ray flare from the Crab Nebula in September 2010, with a sub-day timescale, and a peak emission beyond the steady state emission. Four major gamma-ray flares have been detected by AGILE and Fermi from mid-2007 until now.
The discovery of fast and efficient gamma-ray transient emission from the Crab Nebula leads to substantially revise the current models of particle acceleration. The 2007 gamma-ray flare observed by AGILE shows the evidence of two different types (fast and slow) of enhanced gamma-ray emission. The presence of these two components is found also in the Fermi data with a χ² analysis of the whole data set: several episodes above 5 σ from the Crab average emission (beside the major flares) were found, with a duration ranging from few days up to more than one month. I will discuss the evidence of the fast and slow components of enhanced gamma-ray emission in the AGILE and Fermi data, and the four major flares detected by the two telescopes.