Mental models of health risks are the causal beliefs that comprise one’s inference engines for the interpretation and prediction of health and illness experiences and messages. Mental models of health risks can be parsed into a handful of common elements, including beliefs about causes, consequences, and cures as well as identifying information such as symptoms and timing. Mental models research deriving from a risk and decision analysis framework emphasizes exposure sources and pathways as part of causal thinking as well as how interventions may reduce or increase the risk. Mental models can be developed as a function of one’s goals or the problem in a specific context, rather than as coherent, stable knowledge structures in long-term memory. For this reason they can be piecemeal and inconsistent in the absence of expertise or experience with the risk. Derived often by analogy with more familiar risks, mental models of health risks can lead to effective health behaviors but also to costly inaction or misplaced action. Assessing mental models of hazardous processes can contribute to the design of effective risk communications by identifying the concrete information message recipients need to cope with health risks, thereby making or strengthening common-sense links between risk and action representations. Although a wide variety of research methods are used to investigate mental models, achieving this level of specificity requires attention to substantive details. Researchers are beginning to better understand the interactions between mental models of risk and their social, cultural, and physical contexts, but much remains to explore.