We propose a model of money, credit and bubbles, and use it to study the role of monetary policy in managing asset bubbles. In this model, bubbles pop up and burst, generating fluctuations in credit, investment and output. Two key insights emerge from the analysis. First, the growth rate of bubbles, which is driven by agents' expectations, can be set in real or in nominal terms. This gives rise to a novel channel of monetary policy, as changes in the inflation rate affect the real growth rate of bubbles and their effect on economic activity. Crucially, this channel does not rely on contract incompleteness or price rigidities. Second, there is a natural limit on monetary policy's ability to control bubbles: the zero-lower bound. When a bubble crashes, the economy may enter into a liquidity trap, a regime in which agents shift their portfolios away from bubbles - and the credit that they sustain - to money, reducing intermediation, investment and growth. We explore the implications of the model for the conduct of "conventional" and "unconventional" monetary policy, and we use the model to provide a broad interpretation of salient macroeconomic facts of the last two decades.
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