The harmonization of the monetary policy of the European Union (hereafter referred to as the EU) is of crucial importance as it contributes to a better standard of living for citizens through its actions and efforts towards price stability. This thesis aims to analyze the role and harmonization of the EU's monetary policy and its impact on the member states' economies. The main objective of this thesis is to show how the common monetary policy under the leadership of the European Central Bank contributes to economic stability, economic growth and inflation in the Eurozone. This thesis examines the impact on economic activity, particularly inflation, which has brought numerous challenges for countries with weaker economies following the crisis caused by COVID-19. Inflation is also leading to greater inequality between countries, with wealthier nations better able to "survive" high rates of inflation, while poorer countries are likely to sink even lower. To prevent this, the European Central Bank (hereafter referred to as the ECB) has introduced numerous measures and instruments within the eurozone to ensure price stability. In addition, part of the thesis is dedicated to the euro as the single currency of the EU, which all countries have committed to adopting after joining the EU and meeting the necessary criteria for adoption. The introduction of the euro as an official means of payment brings with it certain advantages, but also disadvantages. The advantages include easier price comparisons, price and economic stability, economic growth and much more. The paper also looks at how the Republic of Croatia has adapted to the new currency and what the situation looks like one year after the changeover to the euro.