Yellow fever was the first disease in man shown to be caused by a filterable virus, and research into its immunology over the past 30 years has led to the development of immunization procedures which not only have resulted in enormous social and economic benefits in the endemic area of the disease, but have served as guides to analogous advances in the whole field of virus infection. The various stages in the preparation and control of the Dakar vaccine by the Pasteur Institute at Dakar, and of the 17D vaccine by the Rockefeller Foundation in its New York Laboratories are described in detail and profusely illustrated by photographs. This is followed by a discussion of the methods and results of mass vaccination, based on the immunization of more than 50 million persons in Africa and South America. The advantages and hazards of mouse-brain and chick-embryo viruses are weighed; scarification as a technique is compared with subcutaneous inoculation; and the combination of smallpox and yellow fever immunization is discussed. There is a chapter on international regulation of yellow fever vaccination, and a select bibliography of 378 titles.