To improve the performance of international health organisations, their essential functions must be agreed. This paper develops a framework to discuss these essential functions. Two groups are identified: core functions and supportive functions. Core functions transcend the sovereignty of any one nation state, and include promotion of international public goods (eg, research and development), and surveillance and control of international externalities (eg, environmental risks and spread of pathogens). Supportive functions deal with problems that take place within individual countries, but which may justify collective action at international level owing to shortcomings in national health systems-such as helping the dispossessed (eg, victims of human rights violations) and technical cooperation and development financing. Core functions serve all countries, whereas supportive functions assist countries with greater needs. Focus on essential functions appropriate to their mandate will better prepare international health organisations to define their roles, eg for WHO to focus on core functions and for the World Bank to focus on supportive ones.