Energetic particles take a fundamental part in the physics of solar flares because they carry a large fraction of the energy released. The most direct and quantitative information on the physical processes involved in the production of non-thermal electrons and ions has been taken from the wealth of hard X-ray (HXR) and gamma-ray (GR) observations performed over the last fifteen years. This review intends to summarize statistical, temporal and spectral characteristics of HXR/GR flares which provide strong constraints on the acceleration/transport process(es) at work during solar flares. The main highlights are: (i) there are no distinct classes of flares; (ii) acceleration time-scales are short (1s or less) but continuous acceleration may occur on long time-scales (tens of minutes to hours); (iii) the energy in ions is comparable to the energy in electrons though there is some variability of electron versus ion content from flare to flare and within one given flare and (iv) there is a preferential acceleration of heavy ions. At present, the different models of particle acceleration have not reached the point where they can account for the ensemble of observational constraints. However, the “avalanche” theory may provide a potential way to progress towards a global understanding of flare energy release and acceleration processes.