Many decisions involve an intuitive assessment of risk; this subjective risk assessment is usually called risk perception. Risk assessment is also a formalized approach to evaluating risk, often defined as a function of the probability and magnitude of loss or harm from an event. Risk assessment is often thought of as ethically obligatory, but since it can be done in more than one way, it is itself subject to ethical assessment.

Risks are routinely assessed formally for a wide variety of human endeavors, from drinking tap water to operating nuclear power plants; for natural hazards such as earthquakes, hurricanes, and floods; and for the human use of and exposure to chemicals and other substances such as arsenic or phthalates. Risks may also be defined and assessed in terms of specific harms or losses to people, for example a person's lifetime risk of dying of heart disease...