This research investigated the phenomenon of expertise development in high school choirs. It sought to determine factors likely to be conducive to the achievement of choral excellence by exploring the principal research question: What is the nature of excellence in the advanced high school choir and how it is established? Excellence in this study refers to "reputational excellence," which was determined through a track record of professional recordings, broadcasts, concerts, and touring, in addition to musical qualities such as advanced repertoire, vocal technique, and tonal qualities well above accepted norms. The individual perceptions of students, teachers, parents, and conductors form a significant body of lived experience that was the foundation for this study. Choirs in schools are educative rather than merely social or diversional activities. Cultural-historical activity theory shaped the approach to the study; singers, conductors, choirs, and their school environments were considered a unified framework. The study drew on emerging theory on the nature of individual and collective expertise and its development. Analysis of the extant literature on the nature of choral expertise, leadership in choirs, and assessing the role of the conductor in expertise development was also undertaken. This collective case study focused on three mixed voice high school choirs located in three different countries. The research design used semi-structured interviews, rehearsal and performance observations, and some artefact study (concert programmes, advertising materials, recordings). Data were generated over two site visits to each choir and included multiple interviews with conductors and singers, spread over an 18-month period, and interviews with accompanists, heads of schools I acknowledge the assistance and support of my supervisors, Margaret Barrett