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 which case customs controls enforce such policies. Customs enforcement at borders can also entail collecting excise tax and preventing the smuggling of dangerous or illegal goods. A customs duty is a tariff or tax on the importation (usually) or exportation (unusually) of goods.
In many countries, border controls for arriving passengers at many international airports and some road crossings are separated into red and green channels in order to prioritise customs enforcement.Within the European Union’s common customs area, airports may operate additional blue channels for passengers arriving from within that area.