The implementation process of monetary integration in Europe may be at least partly explained by the perspective of several theories. The paper identifies some of them and tries to figure out the theory with the highest explanatory power and tries to explain from their perspective the debt crisis that hit some member states of the European Monetary Union (EMU). Optimum currency area (OCA) theory is considered the central theory of the monetary integration process in Europe. The alternatives are the theory of optimal control and policy coordination theory. Theories of European integration could also be applied to the monetary integration process, mainly liberal intergovernmentalism and neofunctionalism. The study concludes that none of these theories provides a comprehensive guidance on the optimal design of the EMU and the debt crisis in several member states of EMU. The most complete theory remains the optimum currency area, other theories provide complementary explanations of rather partial phenomena in the process of monetary integration.