Cancer risk perceptions are a key predictor of risk-reduction practices, health behaviors, and processing of cancer information. Nevertheless, patients and the general public (as well as health care providers) exhibit a number of errors and biases in the way they think about risk, such that their risk perceptions and decisions deviate greatly from those prescribed by normative decision models and by experts in risk assessment. For example, people are more likely to engage in screening behaviors such as mammography when faced with loss-based messages than gain-framed messages, and they often ignore the base rate of a given disease when assessing their own risk of obtaining this disease. In this article, we review many of the psychological processes that underlie risk perception and discuss how these processes lead to such deviations. Among these processes are difficulties with use of numerical information (innumeracy), cognitive processes (eg, use of time-saving heuristics), motivational factors (eg, loss and regret aversion), and emotion. We conclude with suggestions for future research in the area, as well as implications for improving the elicitation and communication of personal cancer risk.