Methods and ray tracing units are provided for performing intersection testing for use in rendering an image of a 3D scene. A hierarchical acceleration structure may be traversed by: traversing one or more upper levels of nodes of the hierarchical acceleration structure according to a first traversal technique, the first traversal technique being a depth-first traversal technique; and traversing one or more lower levels of nodes of the hierarchical acceleration structure according to a second traversal technique, the second traversal technique not being a depth-first traversal technique. Results of traversing the hierarchical acceleration structure are used for rendering the image of the 3D scene. The upper levels of the acceleration structure may be defined according to a spatial subdivision structure, whereas the lower levels of the acceleration structure may be defined according to a bounding volume structure.