  I present a systematic study of gamma-ray flares in blazars. For this
purpose, I propose a very simple and practical definition of a flare as a
period of time, associated with a given flux peak, during which the flux is
above half of the peak flux. I select a sample of 40 brightest gamma-ray flares
observed by Fermi/LAT during the first 4 years of its mission. The sample is
dominated by 4 blazars: 3C 454.3, PKS 1510-089, PKS 1222+216 and 3C 273. For
each flare, I calculate a light curve and variations of the photon index. For
the whole sample, I study the distributions of the peak flux, peak luminosity,
duration, time asymmetry, average photon index and photon index scatter. I find
that: 1) flares produced by 3C 454.3 are longer and have more complex light
curves than those produced by other blazars; 2) flares shorter than 1.5 days in
the source frame tend to be time-asymmetric with the flux peak preceding the
flare midpoint. These differences can be largely attributed to a smaller
viewing angle of 3C 454.3 as compared to other blazars. Intrinsically, the
gamma-ray emitting regions in blazar jets may be structured and consist of
several domains. I find no regularity in the spectral gamma-ray variations of
flaring blazars.
