Smuggling, importing and exporting illegally. Smuggling is likely to develop whenever and wherever governments charge high tariffs, or customs duties, on goods entering or leaving a country, or prohibit or greatly restrict trade in any goods.

Laws of most nations require entering and, in fewer cases, departing travelers and shipments to pass through a checkpoint called a customshouse, or simply customs, to see that required duties are paid. Smuggling sometimes takes the form of “running”—crossing a national boundary without going through customs. More often, the smuggler comes through customs but attempts to conceal the smuggled goods, or contraband.

Goods that are of high value but small bulk are favored by smugglers. Narcotics and other drugs, diamonds, gold, and watches make up the largest portion of the world's contraband in terms of value. Restrictions and demand for restricted goods vary from country to country; thus what is smuggled varies.

In the United States, narcotics and other drugs, which are in considerable demand but highly regulated, pose the biggest smuggling problem for federal authorities. A good deal of ordinary merchandise is technically smuggled into the United States by under-invoicing—understating the amount or the value—on otherwise legitimate import shipments. This type of smuggling has become a problem second only to the narcotic and drug traffic.

Cigarettes and alcohol, often subject to taxes and prohibitions, are widely smuggled. Many nations regulate the sale of guns and munitions; smugglers in these products are called gunrunners. Stolen goods are often smuggled out of the country in which they were stolen and into a country where they can be sold with less chance of detection.

Several countries restrict the removal of works of art and valuable antiquities; such objects are sometimes smuggled out to sell to dealers and collectors. A literary work may be smuggled out of a country where its publication is forbidden or into a country where its sale is banned.

In the United States, the Customs Service in the Treasury Department and the Drug Enforcement Administration in the Justice Department are concerned with the control of smuggling. The U.S. Coast Guard helps apprehend smugglers. Through Interpol, an international police agency, law enforcers of various nations can cooperate in apprehending smugglers.