In the United States and in most of the world, a recent trend in education has been movement away from single-sex secondary schooling and toward coeducation. Single-sex schools are closing or merging for reasons that often have very little to do with the academic merits of coeducation or single-sex education at the secondary level. This study examines current research comparing these two school types and draws conclusions that call into question the wisdom and educational effectiveness of coeducation. Recommendations offered for future action include: (1) that faculty and administration renew their commitment to the purpose of education, (2) that schools work at creating an environment for learning, and (3) that states encourage and support private single-sex schools. A brief historical overview of coeducation and a discussion of two major studies of the two school types are followed by a 27-item annotated bibliography of research on the topic of gender-composition of secondary schools. This material is presented under broad categories: (1) the effects of coeducational and single-sex schools on the school environment; (2) the effects of coeducational and single-sex schools on academic achievement of students; (3) the effects of coeducational and single-sex schools on overcoming traditional sex-role stereotyping in education; and (4) the effects of coeducational and single-sex schools on the academic performance of female students. A 43-item bibliography of works cited is included. (JB)