Amendments passed as part of the No Child Left Behind Act in 2006 made some forms of single-sex (SS) public education legal in the United States. Proponents offer a host of arguments in favor of such schooling. This chapter identifies and evaluates five broad rationales for SS schooling. We conclude that empirical evidence fails to support proponents' claims but nonetheless suggests ways in which to improve coeducation. Specifically, we (a) show that the purported benefits of SS schooling arise from factors confounded with, but not causally linked to, single-sex composition; (b) challenge claims that biological sex is an effective marker of differences relevant to instruction; (c) argue that sexism on the part of teachers and peers persists in SS contexts; and (d) critique the notion that gender per se "disappears" in SS contexts. We also address societal implications of the use of sex-segregated education and conclude that factors found to be beneficial for students should be implemented within coeducational schools.