The money market industry is in the midst of significant change. With the implementation this month of new Securities and Exchange Commission rules designed to make money market funds (MMFs) more resilient to stress, institutional prime and tax-exempt funds must report more accurate prices reflecting the net asset value (NAV) of shares based on market prices for the funds’ asset holdings, rather than promising a fixed NAV of $1 per share. The rules also permit prime funds, which invest in a mixture of corporate debt, certificates of deposit, and repurchase agreements, to impose fees or set limits on investors who redeem shares when market conditions sharply deteriorate. (Funds investing in government securities, which are more stable, are not subject to the new rules.) These changes, driven by a run on MMFs at the height of the financial crisis, add to earlier risk-limiting rules on portfolio holdings.