  RHESSI measurements relevant to the fundamental processes of energy release
and particle acceleration in flares are summarized. RHESSI's precise
measurements of hard X-ray continuum spectra enable model-independent
deconvolution to obtain the parent electron spectrum. Taking into account the
effects of albedo, these show that the low energy cut-off to the electron
power-law spectrum is typically below tens of keV, confirming that the
accelerated electrons contain a large fraction of the energy released in
flares. RHESSI has detected a high coronal hard X-ray source that is filled
with accelerated electrons whose energy density is comparable to the
magnetic-field energy density. This suggests an efficient conversion of energy,
previously stored in the magnetic field, into the bulk acceleration of
electrons. A new, collisionless (Hall) magnetic reconnection process has been
identified through theory and simulations, and directly observed in space and
in the laboratory; it should occur in the solar corona as well, with a
reconnection rate fast enough for the energy release in flares. The
reconnection process could result in the formation of multiple elongated
magnetic islands, that then collapse to bulk-accelerate the electrons, rapidly
enough to produce the observed hard X-ray emissions. RHESSI's pioneering
{\gamma}-ray line imaging of energetic ions, revealing footpoints straddling a
flare loop arcade, has provided strong evidence that ion acceleration is also
related to magnetic reconnection. Flare particle acceleration is shown to have
a close relationship to impulsive Solar Energetic Particle (SEP) events
observed in the interplanetary medium, and also to both fast coronal mass
ejections and gradual SEP events.
