In a tile-based graphics processor when rendering a tile of a render output, which sub-regions, of a plurality of sub-regions that the tile has been divided into for fragment tracking purposes, fragments generated by the rasterisation stage fall within is determined. Then, for at least one sub-region of the plurality of sub-regions that the tile has been divided into, the processing of fragments for the sub-region of the tile is tracked to determine when the processing of all fragments for the sub-region of the tile has been finished. The writing of rendered fragment data for the sub-region of the tile from the tile buffer to memory is controlled on the basis of the tracking of the processing of fragments for the sub-region of the tile.