Ray tracing systems and computer-implemented methods are described for generating a hierarchical acceleration structure to be used for intersection testing in a ray tracing system. Nodes of the hierarchical acceleration structure are determined, wherein each of the nodes represents a region in a scene, and wherein the nodes are linked to form the hierarchical acceleration structure. Data is stored representing the hierarchical acceleration structure. The stored data comprises data defining the regions represented by a plurality of the nodes of the hierarchical acceleration structure. At least one node of the hierarchical acceleration structure is an implicitly represented node, wherein data defining a region represented by an implicitly represented node is not explicitly included as part of said stored data but can be inferred from said stored data. Also described are ray tracing systems and computer-implemented methods for performing intersection testing in the ray tracing system in which, based on conditions in the ray tracing system, a determination is made as to whether testing of one or more rays for intersection with a region represented by a particular node of a sub-tree is to be skipped. The one or more rays are tested for intersection in accordance with said determination.