"The subject matter of international economics, then, consists of issues raised by the special problems of economic interaction between sovereign states.” (Krugman et al., 2011)Institutional innovation fosters the development of these interactions. Thus, European states, on a social background, try to benefit from the cooperation leverage effect, to turn centuries of conflicts, into an economic asset. Such a construction and its pantagruelian twists and turns, as a fusion of history and empires, not far from Victor Hugo’s utopia “the United-State of Europe”, demonstrates the incredible ingenuity, national heterogeneities develop to refine the paradigm “Europe”.In this thesis, we propose to shed some light on some of these challenges, with regard to the countries which, not half a century ago, marked the fracture of the continent, CEECs. These economies, from transition to developed economy, are an unprecedented case study, to understand international economic challenges. After explaining the general framework, in which this work takes place (Chapter 1), we investigate the impact of the ECB’s monetary policy, on the peripheral countries of the euro zone, i.e., EU non-Euro members. In chapter 2, we construct two groups of countries, depending on their exchange rate regime (fixed or flexible). Drawn upon this construction, using monetary, price and output data, we measure the impact of a monetary shock, impulsed by the ECB, on the CEECs. We find that economic integration induces spillover effects, that influence domestic monetary decisions. As expected, pegged economies are more strongly affected, by the monetary policy of the ECB. However, in both groups of countries, we find that spillovers tend to have less impact, on the volatility of our variables (GDP and prices), over the last decade. We explain this, via a more efficient exchange rate channel, to absorb shocks (in flexible exchange) and an increased credibility of domestic monetary institutions.We highlighted that spillovers effects significantly influence the CEECs, with direct impact upon domestic monetary challenges. The first phase of the transition, during the 90’s, has been hit by high level of both inflation and unemployment. Over the last years, inflation seems to be under control, and we observe relatively low unemployment rate, closed to its natural level. This could suggest that monetary credibility has been restored. However, in the process of accession to the Euro zone, monetary leeway is becoming increasingly restricted.Throughout Chapter 3, we use the well-known Phillips curve, to understand the relationship between the unemployment rate and price developments, as a proxy for the effectiveness of monetary policy. The Baltic States, Slovenia and Slovakia are perfect candidates to measure the impact of changes in exchange rate regimes, during accession to the EA. During the ERM-II, the relationship is negative and significant. However, the EA entry is prima facie evidence of a flattened Phillips curve. We explain this result by the fact that in a monetary union, "small" economies do not have sufficient power, to significantly influence monetary policy decisions.To be fully effective, the single policy of the ECB must confront, relatively homogeneous economies. This homogeneity transcends the monetary dimension and directly affects the real economy (evidenced by the Phillips curve). The impacts of asymmetric shocks are smoothed through the adjustment mechanisms in an optimal union. Among these mechanisms, we highlight the role of the labour market, which requires flexibility (of wages) and increased factor mobility. Chapter 4 analyses regional adjustment mechanisms, after an exogenous employment shock. Using regional NUTS-II data, we build a VAR panel, to understand these mechanisms. (...)