Generally, U.S. law imposes customs duties on all tangible imports, with few exceptions. These exceptions include U.S. goods returned from abroad meeting certain preconditions; previously imported goods meeting certain preconditions; imports subject to special duty free provisions (such as the Generalized System of Preferences, "GSP"); and products returned from outer space. Intangible imports, such as electricity, are not subject to U.S. customs duties.

The duty paid depends on the customs classification of the imported merchandise, the customs valuation of the imported merchandise, and the applicable rate of duty.

In most countries imports are classified for customs purposes in accordance with the international Harmonized System or "HS." The U.S. version is known as the Harmonized Tariff Schedules of the United States ("HTSUS"). Imports are assigned a specific classification number, depending on the nature of the import. Applicable duty rates are associated with each classification number.

Customs valuation determines the value that is assigned to an import for duty and statistical purposes. Like customs classification, in the U.S. and other countries that are members of the World Trade Organization "WTO", customs valuation is determined by an international agreement known as the WTO Agreement on Customs Valuation. Most imports are assigned a value based on their "transaction value," usually the contract price. When this is not possible, alternative valuation methodologies specified in the Valuation Agreement are utilized.

Rules of origin fall into two categories: "preferential" rules having to do with duty reduction or elimination, and "non-preferential" rules relating to marking, government procurement, and other administrative requirements. Preferential rules vary depending on the preference program. For the North American Free Trade Agreement ("NAFTA"), goods that entirely originate in a NAFTA country, or that meet changes in tariff classification requirements, or in special circumstances that meet regional value content rules, qualify as NAFTA goods. Non-preferential rules generally assign origin depending on where goods were last "substantially transformed." However, the WTO is now in the process of developing international rules of origin.