The International Organization for Standardization (ISO) under its standard ISO 3166 has established two-letter (2-alpha ) and three-letter (3-alpha ) codes for the various countries of the world, including independent states, dependent areas, and certain areas of contested jurisdiction or special status. These codes are published and revised when needed by the ISO 3166 Maintenance Agency .

The Internet Assigned Numbers Authority (IANA) assigns two-letter codes for country-coded top-level domains in Internet addresses. These codes are the same as the ISO 2-alpha codes in nearly all cases.

The International Olympic Committee also assigns three-letter codes to its national organizing committees and the teams they send to the Olympic Games. These codes coincide with the ISO codes for many countries, but differ for many others. Worse: a few of the IOC codes conflict directly with ISO codes. For example, ANT was previously assigned by the ISO to Netherlands Antilles and by the IOC to Antigua and Barbuda.

The United Nations maintains a list of Distinguishing Signs of Vehicles in International Traffic; these codes appear in oval-shaped signs displayed on the rear of vehicles. These codes were authorized by the UN's 1949 and 1968 Conventions on Road Traffic. (In the U.S. the use of the oval design is not controlled and vehicles often display similar ovals with a wide variety of non-standard codes.) Many of the vehicle codes created since the adoption of ISO 3166 coincide with either the ISO 2-alpha or 3-alpha codes, but many of the codes, especially the older European ones, do not. In Europe, the vehicle codes are often used in postal addressing, preceding the delivery code. (Complete postal addressing information is available from the Universal Postal Union .)

Three-digit numerical codes for countries are also specified by ISO 3166; these codes are actually provided by the United Nations Statistical Division .

The International Telecommunications Union assigns 1-, 2-, or 3-digit country calling codes for international telephone calls. The calling code "1" is assigned to the entire North American Numbering Plan Area (NANPA), which includes the U.S. and its possessions, Canada, Bermuda, the British Commonwealth nations of the Caribbean, and the Dominican Republic. Within this region the "1" is followed by a NANPA 3-digit area code. Also, code "7" is assigned to a large calling area including the Russian Federation and Kazakhstan. All other countries and territories have their own 2-digit or 3-digit code.

In the following table, Internet codes are from IANA's Root-Zone Whois Index. ISO 2-alpha codes are from the ISO 3166 Maintenance Agency, English country names and code elements. ISO 3-alpha codes are not posted to the Internet by ISO; the codes shown are from the United Nations, Country and Region Codes for Statistical Use. which also provided the numerical codes. Some additional 3-alpha and numerical codes are taken from the CIA Factbook Appendix D (Country Data Codes). IOC codes are from the IOC's National Olympic Committees pages. Vehicle codes are from the Transport Division of the UN Economic Commission for Europe; I have omitted several vehicle codes which appear to be obsolete because they refer to previous names of the territory rather than the present name.