"The mission of the Universal Postal Union is to foster the sustainable development of quality universal, efficient accessible postal services in order to facilitate communication among the people of the world"

from the UPU website

The collection of world stamps held by The Royal Mail Archive is the result of this decision made when the UPU was founded in 1874; it was agreed that postage stamps would be exchanged between the members of the UPU.

The collection comprises all the stamps sent out by the UPU from 1874 until now, together with some examples of imprinted postal stationery up to about 1940 (in some places up to the 1960s). However, it is not complete.

Not all stamps issued by member countries were supplied to the UPU at the time of their production. Neither were all postal administrations members of the UPU. As a result, not every basic stamp is represented in the collection. Detailed varieties such as change of perforation, watermark or even colour are unlikely to be present. Earlier stamps are normally overprinted or perforated SPECIMEN, to prevent re-use. In 1969 the collection was made a public record.

The collection was expanded in the 1990s by the purchase of stamps to fill obvious gaps. These included issues from wartime occupations, or short-lived territories which resulted from the collapse of former empires, such as the Russian Empire in 1917. Also added were stamps from countries represented by agents who had not supplied their issues to the UPU.