Abstract: This paper introduces a simple and robust progressive global illumination algorithm based on photon mapping. Progressive photon mapping is a multi-pass algorithm where the first pass is ray tracing followed by any number of photon tracing passes. Each photon tracing pass results in an increasingly accurate global illumination solution that can be visualized in order to provide progressive feedback. Progressive photon mapping uses a new radiance estimate that converges to the correct radiance value as more photons are used. It is not necessary to store the full photon map, and unlike standard photon mapping it possible to compute a global illumination solution with any desired accuracy using a limited amount of memory. Compared with existing Monte Carlo ray tracing methods progressive photon mapping provides an efficient and robust alternative in the presence of complex light transport such as caustics and in particular reflections of caustics.