The original treaty allowed "overseas" countries to be admitted to the union subject to the agreement of administrations having postal relations with them. The 1878 congress decreed, however, that any country could accede directly to the union merely by unilateral declaration and communication of that declaration to the Swiss government. This system was revised by the Paris congress of 1947, which ruled that applications for membership in the union could be filed only by sovereign states and had to be channelled through the Swiss government. Approval is then required by at least two-thirds of the full membership. At the 1964 Vienna Congress, it was also decided that any member nation of the UN could accede directly to the UPU by a formal declaration addressed to the Swiss government. Since Washington Congress 1989, the government of the country concerned will address it directly to the Director General of the International Bureau, who will notify the member countries of the accession or consult with them on the application for admission, as the case may be.
Dependent territories were granted collective membership by a special postal conference held in Bern in 1876. Membership in the UPU as of June 2002 had reached 189 independent states.