ISO 3166 is a standard published by the International Organization for Standardization (ISO). It defines codes for the names of countries, dependent territories, special areas of geographical interest, and their principal subdivisions (e.g., provinces or states). The official name of the standard is Codes for the representation of names of countries and their subdivisions. It consists of three parts:

• ISO 3166-1, Codes for the representation of names of countries and their subdivisions – Part 1: Country codes, defines codes for the names of countries, dependent territories, and special areas of geographical interest. It defines three sets of country codes:
  • ISO 3166-1 alpha-2 – two-letter country codes which are the most widely used of the three, and used most prominently for the Internet's country code top-level domains (with a few exceptions).
  • ISO 3166-1 alpha-3 – three-letter country codes which allow a better visual association between the codes and the country names than the alpha-2 codes.
  • ISO 3166-1 numeric – three-digit country codes which are identical to those developed and maintained by the United Nations Statistics Division, with the advantage of script (writing system) independence, and hence useful for people or systems using non-Latin scripts.
• ISO 3166-2, Codes for the representation of names of countries and their subdivisions – Part 2: Country subdivision code, defines codes for the names of the principal subdivisions (e.g., provinces or states) of all countries coded in ISO 3166-1.
• ISO 3166-3, Codes for the representation of names of countries and their subdivisions – Part 3: Code for formerly used names of countries, defines codes for country names which have been deleted from ISO 3166-1 since its first publication in 1974.

Die ISO 3166 ist ein Standard für die Kodierung von geographischen Einheiten, herausgegeben von der Internationalen Organisation für Normung (ISO).

ISO 3166 (ICS n° 01.140.30) est une norme ISO de codage des pays divisée en trois parties.

ISO 3166 är en internationell standard för landskoder, korta bokstavskombinationer, som används för att beteckna länder i till exempel domännamn på Internet, framför allt toppdomäner.