A system and method for performing intersection testing of rays in a ray tracing system. The ray tracing system uses a hierarchical acceleration structure comprising a plurality of nodes, each identifying one or more elements able to be intersected by a ray. The system iteratively obtains ray requests, each of which identifies a ray and a node against which the ray is to be tested, and performs intersection testing based on the ray requests. The number of ray requests obtained in each iteration reduces responsive to an amount of memory occupied by information relating to the rays (undergoing intersection testing) increasing.