 After considering that a case satisfies the various preconditions we set for Chevron to apply, a reviewing court must assess whether Congress has directly spoken to the precise question. If and only if congressional intent is clear, that's the end of the inquiry. But if the court determines the statute is silent or ambiguous, the court must go to the second step, which is to defer to an agency's interpretation if it's based on a permissible construction of the statute.