Ecological risk assessment has been defined as “a process that evaluates the likelihood that adverse ecological effects may occur or are occurring as a result of exposure to one or more stressors” (USEPA, 1992). Conceptually, risk assessment is implied by Paracelsus’ observation that “only dose differentiates a remedy from a poison.” Risk is not an inherent property of a chemical toxicant but rather the product of its toxicity and the exposure received. Hence, the same chemical can show little risk at low exposures, but high risk at high exposures. This basic concept is embedded in our everyday behaviors. Gasoline is potentially very toxic to human beings, but we handle it without a thought on an almost daily basis. Why? Because even though gasoline has high toxicity, we have decided, consciously or unconsciously, that there is little chance that we would be exposed to that gasoline in a way and quantity that would cause us serious harm. It is this weighing of exposure and potential effect that is, in essence, a form of risk assessment.