Abstract: Montenegro and Serbia are on track to join the EU, followed by other Western Balkan countries. The purpose of this study is to demonstrate these countries’ economic integration maturity. The analysis covers the period between 2006 and 2019. The main questions concern the evolution of Western Balkan countries during the last 15 years, the results that have already been achieved, and the weakest points of their potential accession. The paper suggests that, although these countries are about to fulfil the economic criteria, they are not fully prepared to join the EU from an economic perspective as they face significant challenges in terms of integration maturity. To endorse this suggestion, the paper forms a new composite indicator to provide a comprehensive understanding of the recent development of these countries’ functioning market economy, competitiveness, macroeconomic stability, convergence, and financing ability. Results can contribute to the integration theory and enlargement decision-making.