Solar flares, which have significant space weather consequences, are natural particle accelerators and one of the most spectacular phenomena of solar activity. RHESSI is the most advanced solar X-ray and gamma-ray mission ever flown and has opened a new era in solar flare research following its launch in 2002. This book offers a glimpse of this active research area from a high-energy perspective and contains a comprehensive guideline for RHESSI data analysis. Its main theme is the investigation of particle acceleration and transport in solar flares. The strength of this book lies in its well-balanced account of the latest X-ray observations and theoretical models. The observational focus is on the morphology and spectra of imaged X-ray sources produced by nonthermal electrons or hot plasma. The modeling takes the novel approach of combining the Fokker-Planck treatment of the accelerated particles with the hydro-dynamic treatment of the heated atmosphere. Applications of this modeling technique reach beyond the Sun to other exotic environ-ments in the universe, such as extrasolar planetary auroras, stellar flares, and flares on accretion disks around neutron stars and black holes.