Regardless of what comparative-advantage theory may say about the virtues of unrestricted trade, all nations interfere with international transactions to some degree. Tariffs may be imposed on imports—in some instances making them so costly as to bar completely the entry of the good involved. Quotas may limit the permissible volume of imports. State subsidies may be offered to encourage exports. Money-capital exports may be restricted or prohibited. Investment by foreigners in domestic plant and equipment may be similarly restrained.

These interferences may be simply the result of special-interest pleading, because particular groups suffer as a consequence of import competition. Or a government may impose restrictions because it feels impelled to take account of factors that comparative advantage sets aside. It is of interest to note that insofar as goods and services are concerned, the general pattern of interference follows the old mercantilist dictum of discouraging imports and encouraging exports.

A company that finds itself barred from an attractive foreign market by tariffs or quotas may be able to sidestep the barrier simply by establishing a manufacturing plant within that foreign country. This policy of foreign plant investment expanded enormously after World War II, with U.S. companies taking the lead by investing particularly in western Europe, Canada, Asia, and South America. Industry in other developed countries followed a similar pattern, with many foreign companies establishing plants within the United States as well as in other areas of the world.

The governments of countries subject to this new investment find themselves in an ambivalent position. The establishment of new foreign-owned plants may mean more than simply the creation of new employment opportunities and new productive capacity; it may also mean the introduction of new technologies and superior business-control methods. But the government that welcomes such benefits must also expect complaints of “foreign control,” an argument that will inevitably be pressed by domestic owners of older plants who fear a new competition that cannot be blocked by tariffs. This has been a pressing problem for many governments, particularly insofar as investment by U.S. firms is involved, and it is a chief complaint of critics who view globalization as a form of economic exploitation (see neoliberal globalization). Some countries, such as the United Kingdom and Canada, have been liberal in their admissions policy; others, notably Japan, have imposed tight restrictions on foreign-owned plants.

A tariff, or duty, is a tax levied on products when they cross the boundary of a customs area. The boundary may be that of a nation or a group of nations that has agreed to impose a common tax on goods entering its territory. Tariffs are often classified as either protective or revenue-producing. Protective tariffs are designed to shield domestic production from foreign competition by raising the price of the imported commodity. Revenue tariffs are designed to obtain revenue rather than to restrict imports. The two sets of objectives are, of course, not mutually exclusive. Protective tariffs—unless they are so high as to keep out imports—yield revenue, while revenue tariffs give some protection to any domestic producer of the duty-bearing goods. A transit duty, or transit tax, is a tax levied on commodities passing through a customs area en route to another country. Similarly, an export duty, or export tax, is a tax imposed on commodities leaving a customs area. Finally, some countries provide export subsidies; import subsidies are rarely used.

Tariffs on imports may be applied in several ways. If they are imposed according to the physical quantity of an import (so much per ton, per yard, per item, etc.), they are called specific tariffs. If they are levied according to the value of the import, they are known as ad valorem tariffs.

Tariffs may differentiate among the countries from which the imports are obtained. They may, for instance, be lower between countries that have previously entered into special arrangements, such as the trade preferences accorded to each other by members of the European Union.

Tariffs may be imposed in different ways, each of which will have a different effect on the economy of the country imposing them. By raising the prices of imported goods, tariffs may encourage domestic production. As expenditures on domestic products rise, domestic employment tends to do likewise. This is why tariffs are favoured by industries that find themselves pressed by foreign competitors. The tariff may also encourage tendencies toward a monopolistic market structure to the extent that it lessens foreign competition, with a resulting decrease in the incentive to modernize or innovate. Because tariffs increase the price of an imported commodity, they may also reduce its consumption. The decrease in demand could be large enough in relation to the world market to force the price of the import down.