Abstract: We present progressive photon beams, a new algorithm for rendering complex lighting in participating media. Our technique is efficient, robust to complex light paths, and handles heterogeneous media and anisotropic scattering while provably converging to the correct solution using a bounded memory footprint. We achieve this by extending the recent photon beams variant of volumetric photon mapping. We show how to formulate a progressive radiance estimate using photon beams, providing the convergence guarantees and bounded memory usage of progressive photon mapping. Progressive photon beams can robustly handle situations that are difficult for most other algorithms, such as scenes containing participating media and specular interfaces, with realistic light sources completely enclosed by refractive and reflective materials. Our technique handles heterogeneous media and also trivially supports stochastic effects such as depth-of-field and glossy materials. Finally, we show how progressive photon beams can be implemented efficiently on the GPU as a splatting operation, making it applicable to interactive and real-time applications. These features make our technique scalable, providing the same physically-based algorithm for interactive feedback and reference-quality, unbiased solutions.