Until the recent publication of twelve choral compositions, Rebecca Clarke (1886-1979) was known solely as a professional violist and composer of chamber music and art songs. Clarke composed choral music throughout her active period from 1906 to 1944. In 2004, the first study of Clarke's complete compositional output provided an introduction to the choral music, but only covered selected works. The present study traces the development of Clarke's compositional style through chronological analysis of all twelve choral compositions and an incomplete fragment. Clarke's choral music reveals the selection of high quality, expressive texts; exploration of the timbral, registral, and textural potential of unaccompanied choral music; changes in the treatment of all musical elements; the persistent acquisition of new techniques; and reliance on English choral genres including the madrigal, glee, carol, part song and motet. Chapter one establishes Clarke's importance through a survey of publication, criticism, and scholarship. The chapter also examines the society in which Clarke lived and the issues women composers encountered. A biography then reveals that despite obstacles, Clarke tenaciously pursued compositional study, eagerly acquired new techniques, and expressed enthusiasm for each compositional project. Her skill was confirmed by success in competitions and festivals. Throughout her active period, Clarke supported herself as a professional violist who specialized in chamber music, and a busy performing schedule limited her compositional work. Chapter two documents Clarke's formative vocal- and chamber-music experiences and suggests that her thorough knowledge of chamber music influenced her approach to choral composition. The chapter continues with analysis of Clarke's first seven choral works. The first three are well-crafted part songs that demonstrate Clarke's assimilation of basic compositional techniques. The next four show the increasing complexity of Clarke's style that culminates in her mature masterpiece, "He That Dwelleth in the Secret Place of the Most High." Chapter three presents analysis of three choral arrangements, two works for women's voices, and a choral fragment for mixed voices. The last five complete choral compositions confirm elements of Clarke's mature style and demonstrate her interest in exploring the new challenges of choral arranging and writing for women's voices. While Clarke's choral arrangements of her own songs are idiomatic adaptations for unaccompanied, mixed voices, the last three compositions display the diversity of styles Clarke employed in her late works. Chapter four summarizes changes in Clarke's choral style from 1906 to 1944, examines reasons for her obscurity, and raises questions that merit further research. The appendix which follows clarifies Clarke's intentions and illustrates common editorial issues and solutions through comparison of choral manuscripts and published editions.