When the Treaty of Rome took effect at the beginning of 1958, agriculture was subsidized in all six member countries. The various price-support mechanisms differed substantially, as did foreign-trade policies and tariff levels. The cumulative impact of governmental intervention of various kinds over the years had led to major differences in agricultural price levels among the member nations. With the average price of wheat in the six countries in 1959 indexed at 100, the relative price levels in individual countries were as follows: Germany, 108; France, 78; Italy, 108; Belgium, 101; Luxembourg, 119; and the Netherlands, 86. The achievement of common policies in agriculture appeared to be so difficult that the treaty limited itself to setting forth a number of general provisions on which agreement seemed feasible. Despite this, a common agricultural policy was achieved: all tariff and quota restrictions on trade in farm products among member countries were abolished; a common set of tariffs on agricultural imports from non-EEC countries was established; and a common system of price supports took the place of the former national systems.

The price supports required difficult compromises among the member governments because of the differences in their domestic price levels for farm products. The EEC wheat price, for example, was set roughly halfway between the prices of the lowest-cost suppliers in the community, France and the Netherlands, and those of West Germany, which was the highest. France exerted considerable political pressure to persuade West Germany to accept a substantial lowering of the returns to its wheat producers.

Since its inception, the common agricultural policy experienced several fundamental problems, especially recurrent surpluses and conflicts of interest between large- and small-scale producers. Surpluses originated as a result of the price support system, and while this system helped marginal farmers stay in business, it often encouraged more-productive farmers to overproduce, creating surpluses that had to be purchased with EEC funds. It also caused conflicts of interest between net food exporters that received greater relative support and countries that were net food importers (e.g., the United Kingdom); those that imported more than they exported made large contributions to the common policy but received little return in export subsidies and price supports.

Another fundamental aim of the Treaty of Rome was to achieve a general harmonization of national economic policies. The treaty envisaged the working out of common rules covering such matters as competition, taxation, and other economic legislation. It also called for the development of common policies in such areas as foreign trade and transportation. Members were asked to concert their economic policies in the fields of fiscal and monetary policy, balance-of-payments policy, and social welfare.

The EEC remained a leading proponent of economic integration until 1993, when, renamed the European Community (EC), it became the principal component of the European Union (EU), a broader entity seeking economic and political cooperation. The EC was formed by the Maastricht Treaty (formally known as the Treaty on European Union; 1991), which went into force on November 1, 1993. The treaty also provided the foundation for an economic and monetary union, which included the creation of a single currency. The EC remained the principal component of the EU until 2009, when the Lisbon Treaty eliminated the EC and enshrined the EU as its institutional successor.

Governance and representation within the EU occur through a number of institutions, many of which were formed as part of the EEC. Chief among these are the Council of the European Union, a legislative organization that represents member states; the European Parliament, which has legislative and supervisory roles; and the European Commission, an executive body. The Parliament is the only EU institution whose members are elected by the votes of individual citizens of EU nations. Other EU institutions are the Court of Justice, the Court of Auditors, the European Central Bank (which oversees monetary policy and introduced the euro), the Economic and Social Committee, the Committee of the Regions, the European Investment Bank, and the European Ombudsman. In addition to the institutions, the agencies of the European Union are charged with overseeing particular interests, such as occupational safety, training, or social and environmental concerns.

As was true for the EEC, any European state can request membership in the EU. Candidate countries must demonstrate an adherence to the principles of democracy, market economy, and human rights. Acceptance is granted through a unanimous decision by member countries. In 2020 Britain became the first member state to relinquish membership in the EU.