In a highly controversial decision, the German Federal Constitutional Court (FCC) decided that the Public Sector Purchase Program (PSPP) of the European Central Bank (ECB) was ultra vires because it violated the principle of proportionality. The FCC took a procedural approach to proportionality, finding that the ECB had failed to substantiate its balancing assessment. Therefore, the Court considered itself unable to review whether PSPP was substantively proportionate, but announced that it

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... do so in a future case. The judgment raises the fundamental question of how courts should review monetary policy. The article begins by exploring the role of central banks between independence and accountability. Then, it analyzes, in three steps, what role the principle of proportionality should play in the realm of monetary policy. First, I argue that a traditional substantive balancing test as part of proportionality review is not applicable to monetary policy decisions and highlight the pitfalls of the FCC's approach. Second, I claim that the imposed procedural "duty to substantiate" is not suitable to promote the democratic accountability of the ECB. Third, I contend that, therefore, the PSPP ruling creates an impetus for strengthening the ECB's supranational accountability. I suggest that improving the monetary dialogue with the EU Parliament and establishing a comprehensive transparency regime for the ECB could further this goal. Ultimately, I propose that a narrow reading of the PSPP ruling could reconcile the current conflict between the FCC and the ECB and still provide a sufficient level of judicial accountability for the ECB's monetary policy-making.