In the aftermath of the global financial crisis, major transformations in the monetary policies of central banks were experienced. Central banks, focusing only on price stability in the pre-crisis period, have also taken into account financial stability with the crisis. On the other hand, the tools used by the central banks for monetary policy are not unlimited. In particular, with the transformation of the global financial crisis into the European debt crisis, the central banks of the developed countries have resorted to unconventional monetary policies to revitalize the real economy. In this context, the central banks of developed countries have lowered their policy rates to zero by aggressively lowering the interest rates. Thus, with the implementation of unconventional monetary policies, the balance sheets of central banks expanded and the monetary base was greatly increased. The purpose of the central banks at this point was to try to get out of the deflation process due to the shrinking global demand. In this study, the monetary policy applications and results of the Federal Reserve Bank, The European Central Bank, The Bank of England and the Bank of Japan were discussed. Another aim of the study is to draw attention to the discussions on the effectiveness of the monetary policies after the global crisis.