This paper introduces few concepts to help clinicians to get an idea of the health systems of the migrants they care for, on a daily basis. Health systems are complex to analyze, and is an open, dynamic system, facing diverse influences from multiple levels. Recent globalization has complexified an already complex system. The paper first sets some definitions and then describes succinctly the history of international health financing. It eventually compares several countries' health systems, drawing on few health indicators. Despite significant progresses in terms of global health, the gap between countries, and, within countries between wealthier and poorer people, is increasing. This is particularly worrying while the global aim is health equity.