Code 7652, Naval Research Lab., Washington DC 20375

HESSI will provide the high-resolution gamma-ray spectroscopy not available in early missions. In spite of spectral limitations the experiments on SMM, Yohkoh, and the Compton Gamma Ray Observatory have provided the data for fundamental discoveries relating to particle acceleration, transport and energetics in flares and to the ambient abundance of the chromospheric material. These include: 1) enhancement in the concentration of low FIP elements is evident at depths down to the lower chromosphere, 2) a new method for deriving the spectra of accelerated particles near ~10 MeV, 3) energy content of accelerated ions that often exceeds that in accelerated electrons, 4) a concentration of ³He in accelerated particles at the Sun enhanced by a factor of ~1000 over its photospheric value in some flares, 5) an accelerated alpha/p ratio significantly greater 0.1 in several flares and suggestion that ambient He may also exceed that value in a few flares, 5) limits on beaming of accelerated particles, 6) measurements of the positronium fraction and a temperature-broadened 511 keV line width, and 7) a highly variable ion to electron ratio during flares. We discuss a new method applied to SMM and CGRO/OSSE data that allows the spectrum of broadened gamma-ray lines emitted by accelerated heavy ions to be revealed for the first time. This enables measurement of the heavy ion-abundance and transport at the Sun and comparison with measurements of solar energetic particles in space by ACE. Simultaneous observations by CGRO experiments after launch of HESSI provide complementary high-energy capability lost when the mission was reduced to a Small Explorer.