Ecological risk assessors analyze the effects of human actions on the natural environment. Like other risk assessors, they have been concerned primarily with predictive assessments, those that estimate the nature, probability, and magnitude of effects of proposed actions such as the marketing of a new chemical product or the release of a new effluent. However, risk assessors must increasingly perform retrospective assessments, assessments of human actions that were initiated in the past and may have ongoing consequences. Examples include dumping of hazardous waste or the policy of building tall stacks on coal-fired power plants. Biomarkers are most useful in retrospective risk assessments.