This article provides an overview of the epidemiology of yellow fever and the serious adverse effects that may occur following vaccination. Yellow fever is currently a spreading disease that occurs in epidemic outbreaks in Africa and Latin America, with a trend toward urbanization. The main epidemiological parameters of the disease are reviewed. In the absence of specific treatment, vaccination—which is well tolerated in the vast majority of cases—remains the only effective protection. This vaccination, governed by the International Health Regulations, is valid for a period of 10 years. A summary of the characteristics of the neurotropic and viscerotropic effects responsible for rare but sometimes fatal manifestations is presented. No link has been demonstrated between these effects and the 17D strain. The risk factors identified to date are age and thymus dysfunction. Overall, the risk-benefit analysis remains highly favorable to vaccination, but the decision to vaccinate, particularly for individuals over 60 years of age, should be based on the characteristics of the planned trip and regulatory requirements. Research must continue to understand the host-related immune mechanisms involved. (R.A.)