  A substantial fraction of prompt GRB spectra have soft spectral indexes
exceeding the maximum allowed by the synchrotron model alpha_{max}=-2/3. Some
spectra also exhibit very sharp break at E_p, inconsistent with the smooth
synchrotron spectra. These facts pose a serious problem for the ``optically
thin synchrotron'' interpretation of the prompt emission. We review various
models suggested in order to resolve this puzzle.
