When processing primitives in a tile-based graphics processing system in which a render output is sub-divided into a plurality of tiles for rendering, before a primitive is written to a primitive list corresponding to a region of the render output, it is first written to one or more primitive queues allocated to respective regions of the render output. To write the primitives to primitive lists, primitives are written together from a primitive queue allocated to a region of the render output to the primitive list for that region of the render output, in a single primitive list write cycle.