Abstract: Caustics are detailed patterns of light reflected or refracted on specular surfaces into participating media or onto surfaces. In this paper we present a novel adaptive and scalable algorithm for rendering surface and volume caustics in single-scattering participating media at real-time frame rates. Motivated by both caustic mapping and triangle-based volumetric methods, our technique captures the specular surfaces in light-space, but traces beams of light instead of single photons. The beams are adaptively generated from a grid projected from the light source onto the scene's surfaces, which is iteratively refined according to discontinuities in the geometry and photon distribution. This allows us to reconstruct sharp volume caustic patterns while reducing sampling resolution and fill-rate at defocused regions. We demonstrate our technique combined with approximate ray tracing techniques to render surfaces with two-sided refractions as well as multiple caustic light bounces.