A money market fund is a type of mutual fund that invests in debt securities with fast maturities and minimal credit risk. These funds are among the lowest-volatility investments. Income generated by a money market fund can be tax-exempt, depending on the types of securities in which the fund invests. The U.S. Securities and Exchange Commission (SEC) defines three categories of money market funds based on the fund’s investments: government, prime, and municipal. SEC rules f