  The origin of prompt emission in $\gamma$-ray bursts (GRBs) is highly debated
topic. The observed spectra are supposed to play a crucial role in constraining
the location of the emitting region, the strength of the magnetic field and the
distribution of the accelerated particles. The apparent inconsistency of the
prompt emission spectra with the synchrotron radiation scenario has resulted in
considering more complex models. The inclusion of the soft X-ray data (down to
0.5 keV) in GRB spectra have led to the discovery of low-energy breaks in their
spectra. More importantly, the distribution of spectral slopes has been shifted
towards the prediction of the synchrotron radiation scenario if the break is
associated with the synchrotron cooling frequency. We discuss the recent study
that systematically extend the range of investigation down to the optical
domain. It was shown that the optical-to-gamma-rays spectra are consistent with
the synchrotron model. In addition, widely used empirical model made of thermal
and non-thermal components has been tested. We conclude that most of the
spectra are consistent with the synchrotron scenario while the two-component
model faces difficulties to account for the optical radiation in
presence/absence of the contaminating afterglow emission. We comment on the
parameter space of GRB emitting region derived from the best fit parameters of
the synchrotron model. In a basic one-shot particle acceleration model it
corresponds to the quite contrived solutions for the magnetic field strength
($\sim$ 10 G) and for the radius of the emitting region ($R_\gamma \ge 10^{16}$
cm). Possible modifications of the basic model would be necessary to have a
fully consistent picture.
