Why is it that fewer girls than boys enter and stay in school? What may be done to further increase the school enrollment of girls in developing countries? The answers to these questions constitute the central focus of this paper. It first documents sex differences in primary school participation and wastage rates in developing countries. Participation refers to school enrollment and attendance, and wastage to repeating a grade or dropping out of school. National socioeconomic and demographic characteristics and primary school enrollment levels by sex are then compared. Specific educational policies, institutional factors in the school system, and household factors that have been found by educational researchers to contribute to the lower participation and higher wastage rates for girls are then reviewed. Lastly, policy recommendations are made suggesting ways to increase girls' attendance in primary schools throughout the developing world. The emphasis in this paper is on data disaggregated by sex, on factors that can account for sex differentials, and on policies that will enhance girls' access to primary schools. Policy recommendations discussed in this paper will potentially enhance both girls' and boys' opportunities for primary education; the emphasis, however, is upon recommendations that specifically address the problems of girls. (BW)