Risk is a well-known concept in medicine and in epidemiology and its approach intend to be rational and measurable. Risk measurement makes it possible to communicate with a patient or a population the risk of occurrence of an event. However, it is often difficult to estimate accurately the probability of occurrence of an adverse event and there is therefore uncertainty. In addition, the notion of risk is not easy to grasp for most people. The same risk can be perceived very differently from one individual to another and this perception and understanding depends on psychological, social, cultural, historical and political factors. Understanding this social dimension of risk in clinical practice or in public health is essential to implement efficient risk management.