A computer-implemented method of performing intersection testing in a ray tracing system for use in rendering an image of a 3D scene, the method comprising: performing intersection testing to identify intersections between rays and geometry in the 3D scene by traversing a hierarchical acceleration structure, such that there is a transition in traversal behaviour part-way down the hierarchical acceleration structure, by: traversing (S706), for intersection testing, one or more upper levels of nodes (1200 1 ) of the hierarchical acceleration structure according to a first traversal technique, said first traversal technique being a depth-first traversal technique, wherein said traversing one or more upper levels of nodes of the hierarchical acceleration structure according to the depth-first traversal technique comprises using a metric to determine an order in which to descend nodes of the hierarchical acceleration structure; and traversing, for intersection testing, one or more lower levels of nodes (1200 2 ) of the hierarchical acceleration structure according to a second traversal technique, said second traversal technique not being a depth-first traversal technique; wherein results of said traversing the hierarchical acceleration structure are used for rendering the image of the 3D scene. performing intersection testing to identify intersections between rays and geometry in the 3D scene by traversing a hierarchical acceleration structure, such that there is a transition in traversal behaviour part-way down the hierarchical acceleration structure, by: traversing (S706), for intersection testing, one or more upper levels of nodes (1200 1 ) of the hierarchical acceleration structure according to a first traversal technique, said first traversal technique being a depth-first traversal technique, wherein said traversing one or more upper levels of nodes of the hierarchical acceleration structure according to the depth-first traversal technique comprises using a metric to determine an order in which to descend nodes of the hierarchical acceleration structure; and traversing, for intersection testing, one or more lower levels of nodes (1200 2 ) of the hierarchical acceleration structure according to a second traversal technique, said second traversal technique not being a depth-first traversal technique; traversing (S706), for intersection testing, one or more upper levels of nodes (1200 1 ) of the hierarchical acceleration structure according to a first traversal technique, said first traversal technique being a depth-first traversal technique, wherein said traversing one or more upper levels of nodes of the hierarchical acceleration structure according to the depth-first traversal technique comprises using a metric to determine an order in which to descend nodes of the hierarchical acceleration structure; and traversing, for intersection testing, one or more lower levels of nodes (1200 2 ) of the hierarchical acceleration structure according to a second traversal technique, said second traversal technique not being a depth-first traversal technique; wherein results of said traversing the hierarchical acceleration structure are used for rendering the image of the 3D scene. <EOS>