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European Union Enlargement: Key Terms and Resources

This topical guide explores the history and process of accession, enlargement, and integration to the European Union.

Using This Page

This page contains key terms relevant to accession, integration, and enlargement in the European Union. Using the tabbed box below, discover definitions of key terms and books in the UIUC library catalog related to those terms.


All definitions on this page are sourced from European Union Politics, 6th edition (referenced below).

Key Terms

Accession is the process of joining the EU. A country needs to sign an accession treaty with the current EU member states, which defines the accession conditions and any adaptations and adjustments of the EU Treaty.

The Copenhagen criteria are the criteria that states applying to the European Union have to meet in order to join. The criteria were agreed upon at the Copenhagen European Council meeting in 1993.

The criteria are:

- stability of institutions guaranteeing democracy, the rule of law, human rights and respect for and protection of minorities;
- a functioning market economy and the ability to cope with competitive pressure and market forces within the EU;
- the ability to take on the obligations of membership, including the capacity to effectively implement the rules, standards and policies that make up the body of EU law (the ‘ acquis ’), and adherence to the aims of political, economic and monetary union.


Enlargement is the process of expanding the EU geographically to include new member states.

The European Neighborhood Policy (ENP) was developed in 2004 by the European Commission to frame the bilateral policy between the EU and each partner country. The policy aims to avoid the emergence of new dividing lines between the enlarged EU and its neighbors, and to strengthen prosperity, stability, and security.

Integration refers to a dynamic process of change, combining parts of a unified whole. European integration is usually associated with the intensely institutionalized form of cooperation found in Europe after 1951.

A theory of European integration that privileges the role of states. When conceptualizing decision-making mechanisms in the context of the EU, this refers to decisions being made by the member states only, without involvement of the supranational institutions. In the 1990s, Liberal Intergovermentalism grew from the theory to provide an update as the EU expanded towards Eastern Europe, and therefore created the potential for more differentiation and national preferences.

An approach to the study of EU politics that emphasizes the interaction of the many different actors who influence European policy outcomes. This theory does not privilege actors on one level over actors on others, such as what is seen in Intergovernmental and Neofunctionalist theories. Rather, it understands that policy-making involves interaction between supranational, national, regional, local, individual, and transnational actors.

A theory of European integration that views integration as an incremental process, involving the spillover of integration in one sector to others, ultimately leading to some kind of political community. This tends to privilege the European institutions over the individual states, because much of this spillover tends to occur across borders, thus creating transnational networks.