We reason about the design decisions that led to the system architecture of NVIDIA Iray. The scalable parallelization from single devices to clusters of GPU systems required new approaches to motion blur simulation, anti-aliasing, and fault tolerance, which are based on consistent sampling that at the same time enables push-button rendering with only a minimal set of user parameters. We then dive into technical details about light transport simulation, especially on how Iray deals with geometric light sources, importance sampling, decals, and material evaluation in order to be efficient on GPUs. It is remarkable how well the physically based system extends to modern workflows like, for example, light path expressions and matte objects. The separation of material definition and implementation has been key to the superior performance and rendering quality and resulted in the emerging standard MDL (material definition language).