Risk is the chance, within a time frame, of an adverse event with specific consequences. The person abseiling down the front of the city building next to the giant, hostile ear of corn is protesting about the risks of eating genetically modified food (Figure 1.1). The same person is willing to accept the risks associated with descending from a building suspended by a rope. This book explores the risk assessments that we perform every day and introduces some tools to make environmental risk assessments reliable, transparent and consistent.

Risk assessments help us to make decisions when we are uncertain about future events. Environmental risk assessments evaluate risks to species (including people), natural communities and ecosystem processes. Whatever the focus, the risk analyst's job is to evaluate and communicate the nature and extent of uncertainty. To discharge this duty diligently, we need professional standards against which to assess our performance.

Epidemiologists, toxicologists, engineers, ecologists, geologists, chemists, sociologists, economists, foresters and others conduct environmental risk assessments routinely. Yet analysts often select methods for their convenience or familiarity. Choices should be determined by data, questions and analytical needs rather than by professional convention.

Different philosophies of risk influence how risk assessments are conducted and communicated. Societies and science have evolved conventions for communicating uncertainty that do not acknowledge the full extent of uncertainty. This book explores different paradigms for risk assessment. It evaluates how well different tools for risk assessment serve different needs.