A method of reclaiming memory space allocated to a data structure comprising data objects (910-960) linked by identifying pointers, in which the memory allocated to data objects is reclaimed using two systems: a first system (980), by which a selected part of the data structure is traversed by following the pointers, one of at least two identifiers being allocated to the data objects, a first identifier which indicates that the data object has been traversed so that the data objects referenced by the pointers of that data object have been identified, and a second identifier which indicates that the data object is referenced by a pointer, but the data object has not yet been traversed; and a second system (990), by which an individual data object is selected for deletion to enable the associated memory space to be reclaimed. The second system (990) reads the first system identifier for the individual data object, and if the first identifier is present deletes the data object thereby reclaiming the associated memory space. If the second identifier is present, it allocates a third identifier, where the first system (980) operates to reclaim the memory space allocated to data objects having the third identifier.