Customs is an authority or agency in a country responsible for collecting tariffs and for controlling the flow of goods, including animals, transports, personal effects, and hazardous items, into and out of a country. The movement of people into and out of a country is normally monitored by immigration authorities, under a variety of names and arrangements. The immigration authorities normally check for appropriate documentation, verify that a person is entitled to enter the country, apprehend people wanted by domestic or international arrest warrants, and impede the entry of people deemed dangerous to the country.

Each country has its own laws and regulations for the import and export of goods into and out of a country, which its customs authority enforces. The import or export of some goods may be restricted or forbidden. In most countries, customs are attained through government agreements and international laws. A customs duty is a tariff or tax on the importation (usually) or exportation (unusually) of goods. Commercial goods not yet cleared through customs are held in a customs area, often called a bonded store, until processed. All authorized ports are recognized customs areas.