Abstract: Solar flares are powered by an impulsive release of magnetic energy stored in the solar atmosphere. The release of magnetic energy is heating coronal plasma, but as much as half of the released energy goes into particle acceleration. The acceleration mechanisms that provide these efficient conversions of magnetic energy into supra-thermal particles are currently not well understood. In the past years, significant progress has been made on the observational side by space observatories such as RHESSI, Solar Orbiter/STIX, and HXI onboard ASO-S. After an introduction of the topic, I will highlight recent observational results and I will discuss desired future hard X-ray and gamma-ray instrumentations.