A bank promises its clients immediate access to cash. Depositors can redeem their funds on demand at face value—first come, first served. Other short-term creditors can do the same, albeit at varying speeds, by not rolling over their loans. And, households and firms that pay a fee for a credit commitment can take down their loans at will.

For banks that hold illiquid assets, these promises of liquidity on demand are the key source of vulnerability. The same applies to other financial institutions (de facto or shadow banks) that perform bank-like services, using their balance sheets to transform illiquid, longer-maturity, risky assets into liquid, short-maturity, low-risk liabilities.

A bank run occurs when depositors wish to make a large volume of withdrawals all at once. A bank that cannot meet this sudden demand fails. Even solvent banks—those whose assets exceed the value of their liabilities—fail if they cannot convert their assets into cash rapidly enough (and with minimal loss) to satisfy their clients’ demands. A banking panic is the plural of a bank run: when clients run on multiple banks. We call the spread of runs from one bank to others contagion—the same term used to describe the spread of a biological pathogen.

In this primer, we characterize the sources of bank runs and panics, as well as the tools we use to prevent or mitigate them….