A key issue in the expansion of access to priority health services is how best to implement scaling up efforts. In this paper, we explore the relative merits of vertical and horizontal delivery modes; review the literature on the impact of vertical programmes on health systems and on experiences of integrating these programmes; and analyse health interventions in terms of their different modes of delivery within the health system infrastructure. We conclude that expanding access to priority health services requires the concerted use of both modes of delivery, according to the capacity of health systems as it changes over time. Copyright © 2003 John Wiley & Sons, Ltd.