Free trade is a trade policy that does not restrict imports or exports. In government, free trade is predominantly advocated by political parties that hold economically liberal positions, while economic nationalist political parties generally support protectionism, the opposite of free trade.

Most nations are today members of the World Trade Organization multilateral trade agreements. States can unilaterally reduce regulations and duties on imports and exports, as well as form bilateral and multilateral free trade agreements. Free trade areas between groups of countries, such as the European Economic Area and the Mercosur open markets, establish a free trade zone among members while creating a protectionist barrier between that free trade area and the rest of the world. Most governments still impose some protectionist policies that are intended to support local employment, such as applying tariffs to imports or subsidies to exports. Governments may also restrict free trade to limit exports of natural resources. Other barriers that may hinder trade include import quotas, taxes and non-tariff barriers, such as regulatory legislation.

Historically, openness to free trade substantially increased from 1815 to the outbreak of World War I. Trade openness increased again during the 1920s, but collapsed (in particular in Europe and North America) during the Great Depression. Trade openness increased substantially again from the 1950s onwards (albeit with a slowdown during the 1973 oil crisis). Economists and economic historians contend that current levels of trade openness are the highest they have ever been.

Economists are generally supportive of free trade. There is a broad consensus among economists that protectionism has a negative effect on economic growth and economic welfare while free trade and the reduction of trade barriers has a positive effect on economic growth and economic stability. However, in the short run, liberalization of trade can cause unequally distributed losses and the economic dislocation of workers in import-competing sectors.

Two simple ways to understand the proposed benefits of free trade are through David Ricardo's theory of comparative advantage and by analyzing the impact of a tariff or import quota. An economic analysis using the law of supply and demand and the economic effects of a tax can be used to show the theoretical benefits and disadvantages of free trade.

Most economists would recommend that even developing nations should set their tariff rates quite low, but the economist Ha-Joon Chang, a proponent of industrial policy, believes higher levels may be justified in developing nations because the productivity gap between them and developed nations today is much higher than what developed nations faced when they were at a similar level of technological development. Underdeveloped nations today, Chang believes, are weak players in a much more competitive system. Counterarguments to Chang's point of view are that the developing countries are able to adopt technologies from abroad whereas developed nations had to create new technologies themselves and that developing countries can sell to export markets far richer than any that existed in the 19th century.

If the chief justification for a tariff is to stimulate infant industries, it must be high enough to allow domestic manufactured goods to compete with imported goods in order to be successful. This theory, known as import substitution industrialization, is largely considered ineffective for currently developing nations.

The chart at the right analyzes the effect of the imposition of an import tariff on some imaginary good. Prior to the tariff, the price of the good in the world market and hence in the domestic market is P_world. The tariff increases the domestic price to P_tariff. The higher price causes domestic production to increase from Q_S1 to Q_S2 and causes domestic consumption to decline from Q_C1 to Q_C2.