Risk perceptions are only slightly correlated with the expected values of a probability distribution for negative health impacts. Psychometric studies have documented that context variables such as dread or personal control are important predictors for the perceived seriousness of risk. Studies about cultural patterns of risk perceptions emphasize different response sets to risk information, depending on cultural priorities such as social justice versus personal freedom. This chapter reports the major psychological research results pertaining to the factors that govern individual risk perception and discusses the psychometric effects due to people's risk perception and the experience of severe stress. The relative importance of the psychometric context variables, the signals pertaining to each health risks and symbolic beliefs are explained.