Since its inception in the early nineteenth century, the U.S. commercial paper market has grown to become a key source of short-term funding for major businesses, with issuance averaging over $100 billion per day. In the fall of 2008, the commercial paper market achieved national prominence when increasing market stress caused some to fear that, given its size and importance, the market’s failure would sharply worsen the recession. The Department of the Treasury and Federal Reserve enacted programs targeted at providing credit and liquidity to restore investor confidence. The authors review the history of the commercial paper market, describe its structure and key relationships to money market mutual funds, and present a detailed discussion of the crisis in the market, including the resulting Federal Reserve programs.