Health service delivery programs using minimally-trained community-based health workers (CHWs) have been established in many developing countries in recent years. These programs are expected to improve the cost-effectiveness of health care systems by reaching large numbers of previously underserved people with high-impact basic services at low cost. The reported experience with these programs has been mixed, raising questions about whether the community health worker is an optimal vehicle for extending primary health care. This review of six large-scale community-based worker programs suggests that they have succeeded in some of their objectives but not in others. CHWs increase the coverage and equity of service delivery at low cost compared with alternative modes of service organization. However, they do not consistently provide services likely to have substantial health impact and the quality of services they provide is sometimes poor. Large-scale CHW systems require substantial increases in support for training, management, supervision, and logistics. The evidence suggests that, in general, their potential has not been achieved in large routine programs. Further development of these programs is needed to reinforce their successes and assure that they are adequately supported as an integral component of the basic health system.