Much progress has been made in improving access to basic education in recent years, but international support has been less than promised and the "funding gap" to achieve universal primary education remains stubbornly present. This article identifies six interrelated factors that constrain such donor financing. Prioritization of basic education, evidence and advocacy, and aid architecture present the biggest challenges, followed by donor capacity, partner absorptive capacity and demand. We conclude that future success in attracting external financing for basic education will depend on the sector's ability to raise its political profile, make an evidence-based case for support, tap into new donors and improve its aid effectiveness. (Contains 2 tables and 1 figure.)