Understanding how health systems function and react can be quite complex. In order to clarify what health systems are, the present working paper describes the six main foundations of a health system: human resources, finance, health information, governance, service delivery and consumables and technology. Then through concrete examples, the authors show how general health systems can contribute to improve the impact of eye care services. Finally, the authors show how health systems have become a crucial determinant of eye care programmes in developing countries and show how health systems research can generate more evidence that will help eye care managers to make informed decisions.