This research investigates the economic impacts of European Union (EU) membership using the Difference-in-Differences model. The study systematically analyzes data from multiple countries and time periods to determine how joining the EU influences economic performance, focusing on key metrics such as GDP growth, trade and investment flows, and economic convergence. The findings reveal that EU membership generally results in significant economic benefits, including a notable increase in GDP growth rates, improved trade relations, and higher levels of foreign direct investment. These positive effects are particularly pronounced for countries that joined the EU during major enlargement waves, such as the 2004 accession of ten Central and Eastern European countries. However, the study also identifies variability in the magnitude and persistence of these economic benefits. Factors such as external economic shocks and the initial economic conditions at the time of accession play a crucial role in determining the outcomes. For instance, global financial crises and economic recessions can mitigate the positive effects of EU membership, while less developed economies tend to benefit more in terms of economic convergence. The research highlights the importance of EU structural funds and market access in driving economic growth among new member states. These elements are vital in fostering economic stability and enhancing the overall economic performance of countries within the EU framework. The findings contribute to the broader understanding of economic integration and its role in promoting sustainable economic development, serving as a crucial resource for policymakers and stakeholders in countries considering EU membership.