Credit markets, including the market for bank loans, are characterized by imperfect and asymmetric information. These informational frictions can interact with other economic forces to produce periods of credit-market stress, in which intermediation is unusually costly and households and businesses have difficulty obtaining credit. A high level of credit-market stress, as in a severe financial crisis, may in turn produce a deep and prolonged recession. I present evidence that financial distress and disrupted credit markets were important sources of the Great Depression of the 1930s and the Great Recession of 2007–2009. Changes in the state of credit markets also play a role in "garden-variety" business cycles and in the transmission of monetary policy to the economy.