The development of folk music in the United States includes many distinct styles of quartet singing. This study examines the harmonic development of black religious quartet singing, a subgenre of black gospel music. This community-based, church-oriented style of quartet singing became popular in the early 1920s and continues to thrive today. This study applies traditional tonal theory, jazz theory and Schenkerian analysis to quartet performances, and through these methods of analysis, identifies and traces threads of harmonic development through the genre, which define stylistic epochs. Threads of development include use of meter and rhythm, call-and-response techniques, function of the bass voice, chord structures and use of added dissonance, number and function of voice parts, and variation techniques in repetitive song structures. The investigation identifies three styles of black religious quartet singing: folk, jubilee, and gospel. The performances examined include "I'm Praying Humble" (1937) by Mitchell's Christian Singers, "One Day When the Lord Will Call Me" (1942) by the Southern Sons and "Mary Don't You Weep" (1959) by the Swan Silvertones. Additionally, a recent trend of development is identified and labeled neo-jubilee, and a performance of ''Mary" (1988) by the vocal group TAKE 6 represents this trend. Complete transcriptions of these performances arc provided for musical analysis. The influence of big band music on the development of black religious quartet singing is also examined. The course of development in quartet singing is strikingly similar to that of big band music and is observable in the aforementioned threads of harmonic development. Finally, group singing is the focus of this study, and only styles that are a cappella or harmonically independent of instrumental accompaniment are examined. Styles where instrumental accompaniment is a part of the fundamental structure of the performance are not examined.