Under the federal Chevron doctrine, federal courts generally defer to federal agencies' interpretations of the statutes that the agencies are charged with interpreting, so long as the courts think the agencies' interpretations are "reasonable" (even if not the same as what the court would have chosen). That doctrine is highly controversial, and some argue that courts' duty is to interpret laws the way they think is right.

The same question also arises—usually much less prominently—as to whether state courts should defer to state agencies' interpretations of state statutes. The Westlaw Bulletin just reported that last month, the Mississippi Supreme Court essentially rejected Chevron at the state level, in King v. Mississippi Military Dep't:

Article 1, Section 1 of the Mississippi Constitution of 1890 divides the power of state government into three branches and assigns legislative powers to the legislative branch, judicial powers to the judicial branch, and executive power to the executive branch. While writing and passing statutes is the function of the Legislature, ), interpreting statutes once enacted is the role of the judicial branch. If Article 1, Section 1, were not enough to establish the strict separation of powers under Mississippi's Constitution, then Article 1, Section 2, leaves no doubt. It provides as follows: ["]No person or collection of persons, being one or belonging to one of these departments, shall exercise any power properly belonging to either of the others. The acceptance of an office in either of said departments shall, of itself, and at once, vacate any and all offices held by the person so accepting in either of the other departments.["] Miss. Const. art. 1, § 2. Executive-branch agencies must follow statutes and, absent a declaration from the judicial branch regarding an interpretation of a statute, must decide what statutes mean. However, when the interpretation of a statute comes before the courts, and when deference is given to an agency interpretation, we share the exercise of the power of statutory interpretation with another branch in violation of Article 1, Section 2. Pursuant to the foregoing reasoning, we announce today that we abandon the old standard of review giving deference to agency interpretations of statutes. Our pronouncements describing the level of deference were vague and contradictory, such that the deference could be anywhere on a spectrum from "great" to illusory. Moreover, in deciding no longer to give deference to agency interpretations, we step fully into the role the Constitution of 1890 provides for the courts and the courts alone, to interpret statutes. Although not writing of Mississippi's constitutional separation of powers, we find persuasive the reasoning of then-Judge Gorsuch who wrote, in a separate opinion … in Gutierrez–Brizuela v. Lynch (10th Cir. 2016), that, absent judicial deference to administrative agencies' interpretation of statutes, "[C]ourts would then fulfill their duty to exercise their independent judgment about what the law is."

UPDATE: Tom Kamenick (Wisconsin Institute for Law & Liberty) notes that a 4-2 Wisconsin Supreme Court decision (Tetra Tech EC v. Wisc. Dep't of Rev.) reached much the same result a few weeks ago, though the analysis was more complicated and splintered.