The Chevron doctrine commands courts to defer to agency interpretations of statutes that they administer, so long as they're reasonable. In other words, courts do not decide what the meaning of the statutes are, if agencies have the authority to implement that statute. This Chevron deference doctrine is a bedrock principle of administrative law, and it comes from a decision during the Reagan administration, in 1984. There the environmental protection agency, under president Reagan, was trying to deregulate. They were trying to make it easier for businesses to grow and develop The NRDC opposed this move by the Reagan EPA, because it would not be as stringent in enforcing environmental protections. The case made it all the way up to the Supreme Court, and the question was, can an agency just change its position on a question of law with the definition of stationary sources under the clean air act? Or is it stuck with that position that the prior administration made? At the time, I don't think that the Supreme Court thought it was doing anything remarkable. Justice Stevens, writing the opinion for the court said, that an agency can change its position, so long as the statute is ambiguous, and the agency's interpretation is reasonable. Now importantly, what the court said was, it is not our duty or responsibility to provide the best interpretation of the statute. We might not agree with the agency that this is the best interpretation of the statute, so long as it is reasonable, the agency wins. And that started, or clarified a deference doctrine that has controlled for the last three decades, how federal courts review agency actions. One reason why we have a law / fact distinction in judicial review, why courts review more searching legal questions than factual questions is one of comparative expertise. A court is the expert on saying what the law is. Whereas, they're not the experts, especially appellate courts, on determining facts or making policy judgements. And so, we structure judicial review in a way that a court is reviewing de novo, or saying what the law is. And yet, it's deferring to fact finders - to trial courts, to juries, to federal agencies, to presidents, on what factual and policy determination is. In administrative law, there's deference doctrines even with respect to factual questions. And that brings us to one of the law hottest debated questions in administrative law today of Chevron deference, the level of deference the court should give to agency interpretations of statutes.