Abstract: In 1984, the Supreme Court declared that courts should uphold agency interpretations of ambiguous statutory provisions, so long as those interpretations are reasonable. The Chevron framework, as it is called, is now under serious pressure. Current debates can be both illuminated and softened with reference to an old distinction between interpretation on the one hand and construction on the other. In cases of interpretation, judges (or agencies) must ascertain the meaning of a statutory term. In cases of construction, judges (or agencies) must develop implementing principles or specify a statutory term. In cases that involve statutory construction, the argument on behalf of Chevron is very powerful; agencies have relevant comparative advantages in developing implementing principles. With respect to statutory interpretation, the argument on behalf of Chevron is more controversial. Those who reject Chevron in the context of interpretation should nonetheless accept it in the context of construction. The distinction between interpretation and construction explains some important cases in the 1940s and also in the post-Chevron era.