In a graphics processing system, a command list reader first reads-in and identifies a portion of a command list. The primitives corresponding to the commands in the portion of the command list are then subjected to a Z-only processing pass. This allows a Z-buffer 16 and a hierarchical Z-pyramid 8 to be filled with the appropriate Z-values for the primitives in the portion of the primitive list. The primitives are then subjected to a second, normal rendering, processing pass, but the filled Z-buffer 16 and Z-pyramid from the first processing pass are used together with the Z-tests 7, 10 performed in the second processing pass to determine which primitive should be processed for each sampling point in the second processing pass.