A method of rendering a sub-primitive in a graphics processing system (300; 1100; 1200), wherein the sub-primitive is derivable from one or more input graphics data items via a sequence of one or more processing stages, and wherein a cache is configured for storing a hierarchy of graphics data items, said hierarchy including one or more of the input graphics data items and one or more graphics data items representing results of processing stages of the sequence, the method comprising: determining whether the sub-primitive is stored in the cache; if it is determined that the sub-primitive is stored in the cache, retrieving the sub-primitive from the cache and rendering the retrieved sub-primitive; if it is determined that the sub-primitive is not stored in the cache, determining whether one or more graphics data items of a higher level of the hierarchy are stored in the cache, wherein the sub-primitive is derivable from said one or more graphics data items of the higher level of the hierarchy; and if it is determined that said one or more graphics data items of the higher level of the hierarchy are stored in the cache: retrieving the one or more graphics data items of the higher level of the hierarchy from the cache; deriving the sub-primitive using the retrieved one or more graphics data items of the higher level of the hierarchy; and rendering the derived sub-primitive; characterised in that the method further comprises: maintaining a reference count for each of a plurality of sets of one or more graphics data items, wherein a reference count for a respective set of one or more graphics data items indicates the number of sub-primitives currently being processed which are descendants of a graphics data item of the respective set of one or more graphics data items, incrementing the reference counts for all the graphics data items required to generate a sub-primitive when the sub-primitive is sent for processing; decrementing the reference counts for all the graphics data items required to generate a sub-primitive after the sub-primitive has been processed; and using the reference count for a set of one or more graphics data items to determine whether a graphics data item of the set can be safely evicted from the cache. determining whether the sub-primitive is stored in the cache; if it is determined that the sub-primitive is stored in the cache, retrieving the sub-primitive from the cache and rendering the retrieved sub-primitive; if it is determined that the sub-primitive is not stored in the cache, determining whether one or more graphics data items of a higher level of the hierarchy are stored in the cache, wherein the sub-primitive is derivable from said one or more graphics data items of the higher level of the hierarchy; and if it is determined that said one or more graphics data items of the higher level of the hierarchy are stored in the cache: retrieving the one or more graphics data items of the higher level of the hierarchy from the cache; deriving the sub-primitive using the retrieved one or more graphics data items of the higher level of the hierarchy; and rendering the derived sub-primitive; retrieving the one or more graphics data items of the higher level of the hierarchy from the cache; deriving the sub-primitive using the retrieved one or more graphics data items of the higher level of the hierarchy; and rendering the derived sub-primitive; characterised in that the method further comprises: maintaining a reference count for each of a plurality of sets of one or more graphics data items, wherein a reference count for a respective set of one or more graphics data items indicates the number of sub-primitives currently being processed which are descendants of a graphics data item of the respective set of one or more graphics data items, incrementing the reference counts for all the graphics data items required to generate a sub-primitive when the sub-primitive is sent for processing; decrementing the reference counts for all the graphics data items required to generate a sub-primitive after the sub-primitive has been processed; and using the reference count for a set of one or more graphics data items to determine whether a graphics data item of the set can be safely evicted from the cache. maintaining a reference count for each of a plurality of sets of one or more graphics data items, wherein a reference count for a respective set of one or more graphics data items indicates the number of sub-primitives currently being processed which are descendants of a graphics data item of the respective set of one or more graphics data items, incrementing the reference counts for all the graphics data items required to generate a sub-primitive when the sub-primitive is sent for processing; decrementing the reference counts for all the graphics data items required to generate a sub-primitive after the sub-primitive has been processed; and using the reference count for a set of one or more graphics data items to determine whether a graphics data item of the set can be safely evicted from the cache.