The European Union (EU) has been transforming and evolving throughout the years of its operating. The two processes of deepening and enlarging have had an impact on each other. Since the very beginning the EU has experienced enlargements but one in particular, in the opinion of many scholars, has influenced the organization the most. The enlargement in 2004 by an additional 10 Member States (initially prepared for 12 members) forced the Union to undertake reforms of its treaties and institutions, including the way of decision-making; it also redirected budgetary transfers and impacted on the functioning of the EU’s internal market by expanding its geographical scale and increasing the number of enterprises and consumers that joined it; finally, it gave the EU a new political and geopolitical position. The main aim of the article is to analyze the economic development of the Member States (and especially) the new Member States – herein after referred to as NMS) of this particular enlargement and the EU. The methods of analyses are based on secondary data analysis and the author’s own research related to e.g. regional policy-making.