I first review: a) the current state of knowledge of ion acceleration in solar flares; b) the physics of positron production and annihilation; and c) recent RHESSI data on solar flare annihilation radiation. I then show how the modeling of the positron production and annihilation in the chromosphere, coupled with the newly available high-resolution data on the 511 keV annihilation line, can have important physical implications w. r. t. the models: a) information on the temperature and density of the chromosphere; b) constraints on some of the physical characteristics of the flare and to some extent on the acceleration process.

Although I do mention past instruments (SMM and Yohkoh), this review focuses on the RHESSI satellite, considering the quantum leap it has constituted in the quality of the data it is providing and consequently the constraints it can place on models (of ion acceleration, annihilation environment, etc.).