The World Health Organization (WHO) was established in 1946, as an essential step in the construction of the post-war system of international cooperation. Its creation is the culmination of an historical process that began in the nineteenth century and developed through the establishment in the twentieth century of a number of international sanitary bodies. WHO was meant, in the intentions of its founders, as the central international organization in the area of international health cooperation, with remarkable functions and powers to ensure guidance and coordination of international health work at the global level. It definitely represents the embodiment of the concept that diseases do not know or respect boundaries and that, as the preamble to WHO'S Constitution states, "the achievement of any State in the promotion and protection of health is of value to all". WHO has used its authority and implemented its constitutional mandate in many different and sometimes innovative ways, at the normative, policy-making and technical levels alike. It has become an essential protagonist in the effort of the international community to control diseases and to promote good physical and mental health. It has also become a reference point not only for its Member States but also for the many groups and civil society organizations active in the field of public health. Notwithstanding its importance and achievements, WHO is probably not so well-known outside its specific field of competence as other international agencies, especially as regards its structure and its normative and policy work. The authors, a former legal counsel of WHO and a senior official of WHO'S legal office, have written a thorough and systematic review of WHO in its changing historical and political context, aiming in particular at practitioners and scholars without a specific medical background.