Solar flares are efficient particle accelerators and prime laboratories for studying astrophysical acceleration processes, with a high fraction of the released magnetic energy being carried by energetic particles. Over the last decade, our understanding of particle acceleration and transport in flares has been enhanced by new models and available multi-wavelength observations from X-rays to (E)UV to radio. However, many questions remain about how and where energetic particles are accelerated, and how different plasma environments (e.g., collisions, turbulence) affect the transport and observed properties of energetic particles. In this talk, I will review some of these advances and also discuss upcoming X-ray stereoscopic observations with Solar Orbiter/STIX and new X-ray missions at Earth.