The financial crisis proved strikingly that the structure of the financial system and financial frictions play a crucial role for the effectiveness of monetary policy but also for system risk. Policymakers have overlooked financial intermediation and financial stability. Shadow banks and especially in the euro area the interbank market play a crucial role in propagating financial turmoil. This dissertation addresses these circumstances and contributes to the research on the optimal design of macroeconomic policy with a particular focus on monetary unions with heterogeneous financial sectors. As the consequences for monetary policy are at the heart of this thesis, I use state-of-the-art dynamic stochastic general equilibrium models and implement financial intermediation and frictions to analyze the transmission channels and interactions of (optimal) fiscal, monetary, macroprudential as well as unconventional monetary policy.