A functioning referral system is generally considered to be a necessary element of successful Safe Motherhood programmes. This paper draws on a scoping review of available literature to identify key requisites for successful maternity referral systems in developing countries, to highlight knowledge gaps, and to suggest items for a future research agenda. Key online social science, medical and health system bibliographic databases, and websites were searched in July 2004 for evidence relating to referral systems for maternity care. Documentary evidence on implementation is scarce, but it suggests that many healthcare systems in developing countries are failing to optimise women's rapid access to emergency obstetric care, and that the poor and marginalised are affected disproportionately. Likely requisites for successful maternity referral systems include: a referral strategy informed by the assessment of population needs and health system capabilities; an adequately resourced referral centre; active collaboration between referral levels and across sectors; formalised communication and transport arrangements; agreed setting-specific protocols for referrer and receiver; supervision and accountability for providers' performance; affordable service costs; the capacity to monitor effectiveness; and underpinning all of these, policy support. Theoretically informed social and organisational research is required on the referral care needs of the poor and marginalised, on the maternity workforce and organisation, and on the implications of the mixed economy of healthcare for referral networks. Clinical research is required to determine how maternity referral fits within newborn health priorities and where the needs are different. Finally, research is required to determine how and whether a more integrated approach to emergency care systems may benefit women and their communities.