The economic crisis in Greece led to discussions on whether the euro was secure. To answer this question, first the functions of money, the theoretical basis of monetary policy, and the criteria for the security of a currency must be discussed. The crisis demonstrates that neither Keynesian interest control nor the monetarisms control of the money amount can solve the problem that led up to the crisis. The new approach of monetary theory discussed here is based on equilibriums of goods, money, and labor markets. The found equilibrium solution is not identity (approach of monetarism) and not "equilibrium with unemployment" (Keynes). But the approach in this book leads to a secure currency.