A central principle of modern administrative law is that federal agencies—not the courts—are the primary interpreters of ambiguous federal statutes that Congress has charged the agencies to administer. The Supreme Court crystallized this deference doctrine in its 1984 Chevron decision, though some variation had existed since the 1940s (and maybe even longer, or perhaps not). In 2005, Justice Thomas framed Chevron’s practical significance:
If a statute is ambiguous, and if the implementing agency’s construction is reasonable, Chevron requires a federal court to accept the agency’s construction of the statute, even if the agency’s reading differs from what the court believes is the best statutory interpretation.
That opinion, National Cable and Telecommunications Association v. Brand X Internet Services, upheld and expanded the doctrine of Chevron deference. But in recent years Justice Thomas has led the way in expressing skepticism about it.