The resuits of the Hard X-Ray Imaging Spectrometer on the Solar Maximum Mission are reviewed. Particular emphasis ís given to those results which address phenomena which cannot easily be studied by the other instruments. Among the important discoveries are the following: it has been established that during the onset of the impulsive phase the hard X-rays are emitted from the footpoints of the flare loop, cospatial with the Hα kernels; very large, coronal loops, often containing plasma with temperatures up to 10^K,
are probably quasi-permanent features of the solar atmosphere; there are often soft X-ray precursors from points separated by ^ 10^ km which signal the onset of coronal mass ejections; following the onset of the impulsive phase hot plasma is observed to rise into the corona, sometimes inhibiting further energy input to the chromosphere; surges and prominences may be emitting weakly in soft X-rays during certain phases of their evolution; rapidly-varying
(^ 10‘s) soft X-ray spikes are occasionally seen, with unpredictable corresponding hard X-rays; and there has been a tentative identification of photospheric albedo, backscattered from the hard X-ray emitting plasma as it rises into the low corona.