This chapter discusses the provisions of the statute that are related to aquatic toxicity issues; the implementation of the statute through regulations, policy, and guidance issued by the US Environmental Protection Agency (EPA); the Agency’s implementation of aquatic toxicity activities under the Clean Water Act; the states’ implementation of aquatic toxicity requirements; and EPA’s oversight of the states. The level of technology-based controls applicable to new sources is new source performance standards, which are based on best available demonstrated technology. The policy indicates that, in addition to enforcing specific numerical criteria for toxic pollutants, EPA will use biological techniques or whole-effluent toxicity testing as a complement to chemical-specific analysis to assess effluent discharges and express permit limitations. The Clean Water Act provides statutory authority for EPA to establish provisions regarding aquatic toxicity. Finally, EPA is initiating action to view the integrity of the water environment holistically and is initiating action to develop ecological criteria guidance.