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 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.