Municipal bonds (munis) are issued by states, cities, or other local government agencies. They may be general obligations of the issuer or secured by specified revenues, like fees paid by tollway users. The interest on municipal bonds is usually exempt from federal income taxes. Investors have long regarded these bonds as a relatively safe investment. Not coincidentally, holdings of municipal securities (or munis) have been heavily concentrated among household investors, who own about two-thirds of the $2.9 trillion market.