海关是一个负责征收及保护关税，控制包括动物、财物和危险品出入境物流的国家机关。根据当地的法律规定，某些货物的进出口可能会受到限制或禁止，报关代理执行这些法规。在大多数国家，海关都获得了政府的授权和国际法的许可。

海关关税是货物入境（常见）或出境（不常见）时收的费或税。在英国，关税是国王衡常收入中具有代表性的部分，因此不像消费税、土地税和其他类型的税收那样，需要议会同意才能征收。

尚未清关的商品被监存于通常被称为保税仓库的关税区内，直至完成清关手续。所有授权口岸被确认为关税区。

关境与一国的国境可能相同也可能不同，如欧洲联盟的关境就是整个欧盟的边境。

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.