1. A method of performing occlusion queries in a graphics processing system, the method comprising: when an occlusion query is to be performed: allocating an occlusion counter for the occlusion query in a main memory of or associated with the graphics processing system; providing in a local memory of or associated with the graphics processing system, a local occlusion counter to be used for the respective occlusion query; and using the local occlusion counter in the local memory to count results of the occlusion query; and once the local occlusion counter in the local memory has counted results of the occlusion query, writing the counter value for the local occlusion counter from the local memory to the allocated occlusion counter for the occlusion query in a main memory of or associated with the graphics processing system.