THIS CASEBOOK contains a selection of U. S. Court of Appeals decisions that analyze, interpret, and discuss the doctrine of Chevron deference in environmental law cases. Volume 1 of the casebook covers the District of Columbia Circuit and the First through the Fifth Circuit Court of Appeals. * * * "We evaluate challenges to an agency's interpretation of a statute that it administers within the two-step Chevron deference framework." Catskill Mountains Chapter of Trout Unlimited, Inc. v. EPA, 846 F.3d 492, 507 (2d Cir. 2017) (citing Lawrence + Mem'l Hosp. v. Burwell, 812 F.3d 257, 264 (2d Cir. 2016)); see Chevron U.S.A. Inc. v. Nat. Res. Def. Council, Inc., 467 U.S. 837, 842-43, 104 S.Ct. 2778, 81 L.Ed.2d 694 (1984). At Chevron Step One, we ask "whether Congress has directly spoken to the precise question at issue." Chevron, 467 U.S. at 842, 104 S.Ct. 2778. If Congress's directive is unambiguous, both the agency and the courts are bound by that mandate. Id. at 842-43, 104 S.Ct. 2778. If, instead, "the statute if silent or ambiguous with respect to the specific issue," the analysis proceeds to Chevron Step Two. Id. at 843, 104 S.Ct. 2778; see also Catskill Mountains, 846 F.3d at 507. At that step, "the question for the court is whether the agency's answer is based on a permissible construction of the statute." Chevron, 467 U.S. at 843, 104 S.Ct. 2778.In evaluating reasonableness at Chevron Step Two, "we will accord deference to the agency's interpretation of the statute so long as it is supported by a p.170 reasoned explanation, and 'so long as the construction is a reasonable policy choice for the agency to make.'" Catskill Mountains, 846 F.3d at 507 (quoting Nat'l Cable & Telecomms. Ass'n v. Brand X Internet Servs., 545 U.S. 967, 986, 125 S.Ct. 2688, 162 L.Ed.2d 820 (2005)). Because "a statute's ambiguity constitutes an implicit delegation from Congress to the agency to fill in the statutory gaps," the agency's interpretation must only be reasonable, and need not be the sole permissible or even most reasonable interpretation of the statute. Id. at 520 (quoting FDA v. Brown & Williamson Tobacco Corp., 529 U.S. 120, 159, 120 S.Ct. 1291, 146 L.Ed.2d 121 (2000)); see also Entergy Corp. v. Riverkeeper, Inc., 556 U.S. 208, 218, 129 S.Ct. 1498, 173 L.Ed.2d 369 (2009). Natural Resources Defense Council, Inc. v. US EPA, 961 F. 3d 160 (2nd Cir. 2020)