In this article, we survey the literature that studies fixed-income trading rules and outcomes, including Treasury securities, corporate and municipal bonds, and structured credit products. We compare and contrast the microstructure and regulation of fixed-income markets with equity markets. We highlight the nature of over-the-counter trading in the face of search costs and the associated slow evolution of electronically facilitated intermediation. We discuss the databases available to study fixed-income microstructure, as well as measures and determinants of trading costs, and the important roles dealer networks and limited transparency play. We also highlight unresolved issues.