This article considers the critical roles of risk and risk assessment in the management of health emergencies and disasters. The Task Force on Quality Control of Disaster Management (TFQCDM) has defined risk as the "objective (mathematical) or subjective (inductive) probability that something negative will occur (happen)". Risks with the greatest relevance to health emergency management include: (1) the probability that a health hazard exists or will occur; (2) the probability that the hazard will become an event; (3) the probability that the event will lead to health damage; and (4) the probability that the health damage will lead to a health disaster. The overall risk of a health disaster is the product of these four probabilities. Risk assessments are the tools that help systems at risk-healthcare organizations, communities, regions, states, and countries-transform their visceral reactions to threats into rational strategies for risk reduction. Type I errors in risk assessment occur when situations are predicted that do not occur (risk is overestimated). Type II errors in risk assessment occur when situations are not predicted that do occur (risk is underestimated). Both types of error may have serious, even lethal, consequences. Errors in risk assessment may be reduced through strategies that optimize risk assessment, including the: (1) adoption of the TFQCDM definition of risk and other terms; (2) specification of the system at risk and situations of interest (hazard, event, damage, and health disaster); (3) adoption of a best practice approach to risk assessment methodology; (4) assembly of the requisite range of expert participants and information; (5) adoption of an evidence-based approach to using information; (6) exclusion of biased, irrelevant, and obsolete information; and (7) complete characterizations of any underlying fault and event trees.