Hierarchical acceleration structures with implicitly represented nodes are used for intersection testing in a ray tracing system. Nodes of the hierarchical acceleration structure each represents a region in a scene and are linked to form the hierarchical acceleration structure. Data is stored representing the hierarchical acceleration structure including 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 represented by an implicitly represented node is not explicitly included as part of the stored data but can be inferred from the stored data. Intersection testing in the ray tracing system is performed 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 the determination.