The U.S. Environmental Protection Agency (EPA or the Agency) has developed a new statistical approach that assesses the whole effluent toxicity (WET) measurement of wastewater effects on specific test organisms ability to survive, grow, and reproduce. This new approach is called the Test of Significant Toxicity (TST) and is a statistical method that uses hypothesis testing techniques based on research and peer-reviewed publications. The hypothesis test under the TST approach examines whether an effluent, at the critical concentration (e.g., in-stream waste concentration or IWC), as recommended in EPAs Technical Support Document (TSD; USEPA 1991) and implemented under EPAs WET National Pollutant Discharge Elimination System (NPDES) permits program, and the control within a WET test differ by an unacceptable amount (the amount that would have a measured detrimental effect on the ability of aquatic organisms to thrive and survive). Since the inception of EPAs NPDES WET program in the mid 1980s, the Agency has striven to advance and improve its application and implementation under the NPDES WET Program. The TST approach explicitly incorporates test power, which, using the TST approach, is the ability to correctly classify the effluent as acceptable under the NPDES WET Program (i.e., non-toxic). The TST approach also provides a positive incentive to generate high quality, valid WET data to make informed decisions regarding NPDES WET reasonable potential (RP) and permit compliance determinations. Once the WET test has been conducted (using multiple effluent concentrations and other requirements as specified in the WET test methods), the TST approach can be used to analyze valid WET test results to assess whether the effluent discharge is toxic. The TST approach is designed to be used for a two concentration data analysis of the IWC or a receiving water concentration (RWC) as compared to a control concentration.