Ecological hazard and risk assessment methods have been developed by the U.S. EPA to systematically evaluate new chemicals, existing chemicals, and genetically engineered microorganisms (OEMs) regulated by the Office of Toxic Substances (OTS) under the Toxic Substances Control Act (TSCA). About 70 000 existing chemicals are already on the TSCA Inventory, with thousands of new chemicals being assessed each year for their eventual manufacture and placement on this inventory. New chemicals have little or no ecological test data to evaluate. This lead to the development and regular use by OTS of Quantitative Structure Activity Relationships (QSARs) to evaluate the potential hazards of new chemicals to aquatic organisms. The estimates of hazard for existing chemicals and OEMs are mostly based upon test data supplied by industry. Over the last decade, OTS has identified several test endpoints of ecological concern, developed a scheme for the tier-testing of chemicals, provided guidelines on the methods for performing such tests, and evaluated and/or developed ecological data used in the assessment of thousands of chemicals. Estimates of aquatic and terrestrial hazards are compared with the concentrations of substances expected in the environment and an evaluation of the potential risk made. Ecological risk methods vary from the simple comparison of the potential hazards with the estimated exposure (that is, the quotient method) to simulation modeling.