This paper proposes that the Mundellian Trilemma remains valid despite the emergence of a world financial cycle. A clear distinction must be made between monetary policy independence and insulation of an open economy’s financial system. A flexible exchange rate allows an optimizing central bank to chart an independent course but does not insulate the domestic economy from foreign monetary or financial shocks. The gains from a flexible exchange rate may be considerable and vary in accordance with the mandate of the central bank. The Mundellian Trilemma highlights the acute shortage of policy instruments. We show that macroprudential policy in the form of an interest equalization tax, enhances the ability of an optimizing central bank to effectively stabilize domestic output and inflation in the presence of policy changes abroad and potentially destabilizing capital flows.