This research project tries to assess the impact of a future European Union enlargement on the economies of the candidate countries from Central and Eastern Europe and on the current members of the Union. The study looks at benefits and costs for the European Union and for the ex-socialist countries. Membership for the Eastern European countries will help promote economic growth and raise standards of living.The costs of membership, however, should not be underestimated since, for Eastern European states, they will be in addition to the already high costs of transitioning from centrally planned economies to market economies. The enlargement will also be beneficial for the EU. In addition to the larger market, an expansion into Eastern Europe will also mean more and easier business for firms because of uniform standards. The economic costs for the Union are mainly associated with the need for an increase in developmental funding. An enlargement will also create a larger European Union with greater political power in international negotiations and will raise security in Europe. The research also examines how the accession of Greece, Portugal, and Spain has influenced the economies of the new and old members of the Union. Portugal and Spain, as well as the European Union, benefited economically from the enlargement. Greece, on the other hand, lagged in its economic development after it joined the European Union. This example shows that Union accession is more likely to have a positive impact for Eastern Europe; however, European Union membership alone will not bring economic growth (as Greece's case shows). The study also looks at economic development in Central and Eastern European countries for the past decade. The analysis shows that European Union accession may benefit Eastern Europe but probably will not have a significant economic impact on the European Union. Should Central and Eastern European countries join the EU? Yes, but they should be cautious and not overestimate the benefits or ignore the costs of membership.