When a programmable execution unit of a graphics processor is executing a graphics processing program to render a frame that represents a view of a scene using a ray tracing process, and the ray tracing process requires the determination of geometry that will be intersected by a ray, the programmable execution unit sends a message to a ray tracing acceleration data structure traversal circuit of the graphics processor, for the ray tracing acceleration data structure traversal circuit to perform a traversal of a ray tracing acceleration data structure for the scene to determine geometry for the scene that may be intersected by the ray. The ray tracing acceleration data structure traversal circuit then returns to the programmable execution unit an indication of geometry that may be intersected by the ray, and the programmable execution unit uses the indicated geometry to determine any geometry that is intersected by the ray.