  We report on BeppoSAX simultaneous X- and gamma-ray observations of the
bright gamma-ray burst (GRB) 990123. We present the broad-band spectrum of the
prompt emission, including optical, X- and gamma-rays, confirming the
suggestion that the emission mechanisms at low and high frequencies must have
different physical origins. We discuss the X-ray afterglow observed by the
Narrow Field Instruments (NFIs) on board BeppoSAX and its hard X-ray emission
up to 60 keV several hours after the burst, in the framework of the standard
fireball model. The effects of including an important contribution of Inverse
Compton scattering or modifying the hydrodynamics are studied.
