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. Traditionally, customs has been considered as the fiscal subject that charges customs duties (i.e. tariffs) and other taxes on import and export. In recent decades, the views on the functions of customs have considerably expanded and now covers three basic issues: taxation, security, and trade facilitation.

Each country has its own laws and regulations for the import and export of goods into and out of a country, enforced by their respective customs authorities; the import/export of some goods may be restricted or forbidden entirely. A wide range of penalties are faced by those who break these laws.

The traditional function of customs has been the assessment and collection of customs duties, which is a tariff or tax on the importation or, at times, exportation of goods. Commercial goods not yet cleared through customs are held in a customs area, often called a bonded store, until processed. Authorized ports are usually recognized customs areas.

A more recent objective of customs has been trade facilitation, which is the streamlining of processing of import and export of goods to reduce trade transaction costs. The contemporary understanding of the “trade facilitation” concept is based on the Recommendation No. 4 of UN/CEFACT “National Trade Facilitation Bodies”. According to its provisions (para. 14),

facilitation covers formalities, procedures, documents and operations related to international trade transactions. Its goals are simplification, harmonization and standardization, so that transactions become easier, faster and more economical than before.

The September 11, 2001 terrorist attacks in the United States has become the cardinal factor in prompting a significant strengthening of the security component of modern customs operations, after which security-oriented control measures for supply chains have been widely implemented for the aims of preventing risk identification. At airports today, customs functions as the point of no return for all passengers; once passengers have cleared customs, they cannot go back. Anyone arriving at an airport must also clear customs before they can officially enter a country. Those who breach the law will be detained by customs and likely returned to their original location. The movement of people into and out of a country is normally monitored by migration authorities, under a variety of names and arrangements. Border control 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 deny the entry of people deemed dangerous to the country.

The most complete guidelines for customs security functions implementation is provided in the World Customs Organization Framework of Standards to Secure and Facilitate Global Trade (SAFE), which has had five editions in 2005, 2007, 2010, 2012, and 2018, respectively.

Customs is part of one of the three basic functions of a government, namely: administration; maintenance of law, order, and justice; and collection of revenue. However, in a bid to mitigate corruption, many countries have partly privatised their customs. This has occurred by way of contracting pre-shipment inspection agencies, which examine the cargo and verify the declared value before importation occurs. The country's customs is obliged to accept the agency's report for the purpose of assessing duties and taxes at the port of entry.