Municipal Bond
A municipal bond — commonly called a 'muni' — is a debt security issued by a state, city, county, or other government entity to finance public projects, and whose interest income is typically exempt from federal income tax.
Municipal bonds occupy a unique niche in the U.S. fixed income market, primarily because of their tax-advantaged status. Interest paid on most munis is exempt from federal income tax and, in many cases, from state and local taxes for residents of the issuing state. This tax exemption makes munis particularly attractive to high-income investors in the top federal tax brackets, who are willing to accept a lower nominal yield in exchange for keeping more of their interest income.
The taxable equivalent yield formula allows investors to compare muni yields directly to taxable bonds: taxable equivalent yield = muni yield / (1 - marginal tax rate). For an investor in the 37% federal bracket, a muni yielding 3.5% is equivalent to a taxable bond yielding approximately 5.56%. This differential explains why high-net-worth individuals and certain institutional investors favor munis despite their lower nominal yields.
There are two broad categories of municipal bonds. General obligation (GO) bonds are backed by the full taxing power of the issuing government — the issuer can raise taxes to repay bondholders — making them generally the safest type of muni. Revenue bonds are backed solely by the revenue generated by a specific project, such as a toll road, airport, water utility, or hospital. Revenue bonds carry more credit risk than GO bonds but often offer higher yields. The $4 trillion U.S. municipal bond market funds a vast range of infrastructure: schools, hospitals, highways, water treatment plants, and airports.
Credit quality varies widely in the muni market. States like Texas and Virginia consistently earn top credit ratings from Moody's and S&P, while others have faced fiscal stress. Detroit's 2013 bankruptcy — the largest U.S. municipal bankruptcy in history — shocked investors who had assumed general obligation bonds were near risk-free. Puerto Rico's multiyear debt restructuring process, which began in 2017 with roughly $70 billion in liabilities, further underscored that muni credit risk deserves serious analysis.
Munis are also subject to the alternative minimum tax (AMT) in certain cases, particularly private-activity bonds used to finance projects with private beneficiaries such as airports or convention centers. Investors must confirm whether a specific muni is 'AMT-exempt' before assuming full tax-free status. Despite these complexities, the muni market remains one of the most important vehicles for tax-efficient income in U.S. portfolio management.