Abstract: Information items commonly have a unique portion and a common portion that is the same for multiple items. The common portions of information items are stored once. Each common portion is assigned a globally unique single-instance identifier. Each unique portion of an information item stores the single-instance identifier of the associated common portion. A reference count, stored with the common portion, reflects the number of unique portions associated with the common portion. When an information item is created or distributed, the single-instance identifier of the common portion is used to ensure that multiple copies of the common portion are not stored.