A postal code (known in various countries as a post code, postcode, or ZIP code) is a series of letters and/or digits appended to a postal address for the purpose of sorting mail. Once postal codes were introduced, other applications became possible.

In February 2005, 117 of the 190 member countries of the Universal Postal Union had postal code systems. Countries that do not have national systems include Ireland and Panama. Although Hong Kong and Macau are now Special Administrative Regions of China, each maintains its own long-established postal system, which does not utilize postal codes for domestic mail, and no postal codes are assigned to Hong Kong and Macau. Mail between Hong Kong, Macau and mainland China is treated as international.

Although postal codes are usually assigned to geographical areas, special codes are sometimes assigned to individual addresses or to institutions that receive large volumes of mail, such as government agencies and large commercial companies. One example is the French CEDEX system.