The health sector has always been an important recipient of global attention and external assistance. Humanitarian concerns about the health of the world’s poor, along with fears about the spread of epidemics such as HIV/AIDS, have made health a central pillar of most development policies. Over the last 30 years attention has increased with the emergence of new health threats - such as HIV/AIDS and pandemic influenza - and with recognition that health is a key determinant of economic growth, labor force productivity and poverty reduction. At the same time, health is increasingly viewed as a human right, the fulfillment of which places obligations on both developed and developing countries. Health has also been at the forefront of the debate on what aid effectiveness means from a sectoral perspective. In the mid-1990s health policymakers designed and pioneered sector-wide approaches (SWAps), which aimed to foster ownership, improve donor harmonization and aid predictability, and align policy behind a health-reform program agreed between government and donors. As this paper illustrates, aid effectiveness is particularly challenging in health. As with other sectors, difficulties are the result of inefficiencies in the global aid architecture and of poor country policies; however, problems in health are exacerbated by the inherent complexities of the sector itself. The large number and diverse nature of development partners active in health, the large unmet needs, the dependency on multiple sectors to achieve health outcomes, the major roles of the private sector in both financing and delivery, and the long-term recurrent nature of most health needs have created challenges for countries, but have also stimulated innovative ideas on how to move forward. As a result, the health sector may be a good "tracer" sector for OECD/ DAC to monitor overall harmonization and alignment progress.