Customs is an authority or agency in a country responsible for collecting and safeguarding customs duties and for controlling the flow of goods including animals, personal effects and hazardous items in and out of a country. Depending on local legislation and regulations, the import or export of some goods may be restricted or forbidden, and the customs agency enforces these rules. The customs agency may be different from the immigration authority, which monitors persons who leave or enter the country, checking for appropriate documentation, apprehending people wanted by international arrest warrants, and impeding the entry of others deemed dangerous to the country.

A customs duty is a tariff or tax on the import of or export of goods. In the Kingdom of England, customs duties were typically part of the customary revenue of the king, and therefore did not need parliamentary consent to be levied, unlike excise duty, land tax, or other forms of taxes.

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 area.