This article assesses 36 peer-reviewed studies of the impact of primary healthcare (PHC) on health outcomes in low- and middle-income countries. Studies were abstracted and assessed according to where they took place, the research design used, target population, primary care measures, and overall conclusions. Results indicate that the bulk of evidence for PHC effectiveness is focused on infant and child health, but there is also evidence of the positive role PHC has on population health over time. Although the peer-reviewed literature is lacking in rigorous experimental studies, a small number of relatively well-designed observational studies and the consistency of findings generally support the contention that an integrated approach to primary care can improve health. A few large-scale experiences also help identify elements of good practice. The review concludes with several recommendations for future studies, including a focus on better conceptualizing and measuring PHC, further investigation into the advantages of comprehensive over selective PHC, need for experimental or quasi-experimental research designs that allow testing of the independent effect of primary care on outcomes over time, and a more detailed conceptual framework guiding overall evaluation design that places limits on the parameters under consideration and describes relationships among different levels and types of data likely to be collected in the evaluation process.