Area codes were developed in 1947 to meet the post-World War II surge in demand for telephone services. The first area code to be used was New Jersey's 201 area code in 1951.

In recent years, the demand for more telephone numbers has exhausted the supply of telephone numbers that are available in certain geographic areas and led to the need for additional area codes. Many new telephone numbers are needed for fax machines, cellular phones, pagers, computer lines and second lines. By adding new area codes, millions of new numbers have become available to accommodate the demand.

Area codes are assigned according to the North American Numbering Plan (NANP). It was invented in 1947 by AT&T and Bell Laboratories. It conforms to the International Telecommunications Union Recommendation E.164, which is the international standard for numbering plans.

The NANP is the numbering plan for the Public Switched Telephone Network in the United States and its territories, Canada, Bermuda, and many Caribbean nations. NANP numbers are ten-digits in length in the following format: NXX-NXX-XXXX, where N is any digit from 2 to 9 and X is any digit from 0 to 9. The first three digits are usually called the area code. The second three digits are called the central office code, or prefix. The final four digits are the line number.

Starting in 1958, several island countries and territories in the Caribbean and adjacent areas of the Atlantic were assigned North American area code 809. Area code 809 was nearing its capacity in the mid-1990's, so new area codes were assigned in 1995, 1996, and 1997, with the end result that each country or separate territory now has its own area code. The Dominican Republic retained area code 809. St. Vincent and the Grenadines has finally ended permissive dialing, as of 1999-05-31. Only the Dominican Republic continues to use area code 809. All numbering (assignment of prefixes) within the Caribbean region of the NANP will now be handled by the telecom ministries of the various countries and territories. Puerto Rico overlaid 939 onto its 787 area code in 2001 and the Dominican Republic overlaid 829 onto its 809 area code in 2005; however, there are not expected to be any further area code splits in the region for many years.