Due to globalization in financial markets, many countries have allowed the free movement of capital and opened their systems to other countries. Within the European Union (EU) this process has been relatively rapid. Control of capital movements was abolished as early as the 1990s. In order to strengthen the EU's monetary policy, the most important is the process of integration among the member states themselves. The EU's high degree of monetary integration rests on the single market, the single monetary policy and the single central bank. The European Central Bank (ECB) pursues monetary policy for the Member States of the European Economic and Monetary Union. Its main goal is price stability. The ECB has many secondary objectives, such as low inflation, high employment, stable growth and development, development of competitiveness of the EU as a whole, etc., which it achieves beyond the primary goal. No other goal should contradict the primary goal. Given that Croatia has accepted the mandatory entry clause in the euro area, the ECB's decisions are also extremely important for the Croatian National Bank (CNB). In this paper, we will deal in more detail with the development of a single EU monetary policy as well as the work of the ECB.