Virtually all major theories of health‐related behaviours are based on the assumption that people estimate their perceived susceptibility to a disease and evaluate the costs and benefits of precautionary behaviour before taking action. Generally, perceived risk or susceptibility is seen as an important determinant of preventive action. First I briefly summarize the literature on the accuracy of perceived risk or susceptibility. Next I turn to the relation between perceived risk and precautionary behaviour. This article reviews the evidence concerning the assumption that perceived risk or vulnerability is an important determinant of precautionary behaviour and points at some shortcomings of the existing literature. Comparative optimism or unrealistic optimism (i.e. the belief that risks apply more to others than to oneself) is also assumed to be related to preventive behaviour. This field of research is briefly reviewed and it is concluded that there is hardly any evidence for the presumed detrimental effect of optimism on preventive behaviour. This is followed by some methodological considerations about how to measure perceived risk and investigate its role as a behavioural determinant. It is recommended to reduce the diversity in which perceived risk is measured and to focus on conditional risk as opposed to unconditional risk. It will be argued that perceived vulnerability is a necessary but not a sufficient condition for preventive action. Other more proximal antecedents of preventive behaviour will be briefly discussed, followed by a brief discussion of the implications for health education practic