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. The 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 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). EPA Regions and their NPDES states can still use EPAs TSD approaches. The TST approach is another statistical option to analyze valid WET test data. 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 Program. The TST approach explicitly incorporates test power (the ability to correctly classify the effluent as nontoxic, also see reference in the glossary under power) and provides a positive incentive to generate valid, high quality 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 EPA WET test methods), the TST approach can be used to analyze the WET test results to assess whether the effluent discharge is toxic at the critical concentration. Performing the EPA WET test where the minimum five required test concentrations (pursuant to the EPA WET test methods) can establish a concentration-response curve. The TST approach is designed to be used for a two concentration data analysis of the IWC or a receiving water concentration (RWC) compared to a control concentration. Using the TST approach, permitting authorities will have more confidence when making NPDES determinations as to whether a permittees effluent discharge is toxic or non-toxic. Use of the TST approach does not result in any changes to EPAs WET test methods; however, a facility might desire to modify its future WET tests by increasing the number of replicates over the minimum required (USEPA 1995, 2002a, 2002b, 2002c) by the approved EPA WET test method to increase test power, which is the probability of declaring an effluent non-toxic if the organism response at the IWC is truly acceptable. If WET tests have already been performed, the WET data generated cannot be modified to increase the number of test replicates because the TST analysis is done on valid WET data generated within a WET test.