The question of whether single-sex schooling is preferable to coeducation for some or all students continues to be hotly debated. Much of the debate is philosophical and would be waged even if single-sex schooling were shown to be highly advantageous for one or more subpopulations. However, the actual research evidence, although suggestive that single-sex schools can benefit some students in some realms of academic and socio-emotional accomplishment, is equivocal. As a result of the obstacles to conducting true randomized experiments, few or no studies have provided definitive evidence for or against single-sex schooling. The primary purpose of this paper is to enumerate, categorize, and critique extant explanations for why single-sex schools should be more or less effective than coeducation schools, in what realms, and for what outcomes. (Contains 2 tables.)