Section 518 of the Clean Water Act (CWA), enacted as part of the 1987 amendments to the statute, authorizes EPA to treat eligible Indian tribes with reservations in a manner similar to states (TAS) for a variety of purposes, including administering each of the principal CWA regulatory programs and receiving grants under several CWA authorities. Since 1991, EPA has followed a cautious interpretation that has required tribes, as a condition of receiving TAS regulatory authority under section 518, to demonstrate inherent authority to regulate waters and activities on their reservations under principles of federal Indian common law. The Agency has consistently stated, however, that its approach was subject to change in the event of further congressional or judicial guidance addressing tribal authority under CWA section 518. Based on such guidance, EPA in the interpretive rule we are finalizing today concludes definitively that section 518 includes an express delegation of authority by Congress to Indian tribes to administer regulatory programs over their entire reservations, subject to the eligibility requirements in section 518. This reinterpretation streamlines the process for applying for TAS, eliminating the need for applicant tribes to demonstrate inherent authority to regulate under the Act and allowing eligible tribes to implement the congressional delegation of authority. The reinterpretation also brings EPA's treatment of tribes under the CWA in line with EPA's treatment of tribes under the Clean Air Act, which has similar statutory language addressing tribal regulation of Indian reservation areas. This interpretive rule does not revise any regulatory text. Regulatory provisions remain in effect requiring tribes to identify the boundaries of the reservation areas over which they seek to exercise authority and allowing the adjacent state(s) to comment to EPA on an applicant tribe's assertion of authority. This rule will reduce burdens on applicants associated with the existing TAS process and has no significant cost.