Although research on the effects of K-12 single-sex education is inconclusive in general, some common themes emerge in the research literature. This Digest reviews that research with particular attention to effects on girls' attitudes and achievement. The Digest first discusses attitudinal variables (i.e., self-esteem and attitudes toward academic subjects) and then explores research on achievement variables. The Digest concludes with a summary of research findings, including the following: (1) studies of attitudinal variables yielded some consistent findings, including differences in specific domains of self-concept between girls in single- and mixed-sex schools, and findings that support the view that single-sex contexts foster less stereotypical views of subjects; (2) studies finding positive achievement effects attributable to school type tend to view their findings as specific to certain contexts and group characteristics (including socioeconomic status); (3) some studies recognize that some single-sex schools are "doing something different" that may be reproducible in the coeducational context--these studies view policy and training interventions as particularly valuable; and (4) other studies have not claimed positive achievement effects for single-sex programs. The research, while inconsistent in its assessments of whether single-sex education is "better" than coeducation for girls, does reveal areas of consensus on specific indicators, which may serve as starting points for further research into how single-sex schools affect educational outcomes. (Contains 12 references.) (LPP)