The classical view of natural law, often traced to Aquinas’ statement that “unjust law is no law at all,” finds few defenders today. Even those most sympathetic to natural law theories do not embrace the classical account, but, instead, convert Aquinas’ claim into a claim of political theory (unjust law does not obligate) or construct new “natural law” accounts about the connection between legal and moral principles in a theory of adjudication. In this paper, I defend the view that extreme injustice disqualifies otherwise valid official directives from counting as “law”. Indeed, I suggest that modern positivism’s characterization of the normative claims that typify legal systems leads inevitably to the conclusion that “law,” as a conceptual matter, must be understood by insiders who employ the term to admit moral limits on what can count as “law.” I proceed as follows. First, I begin with some preliminary clarifying comments about methodology and the precise issue under discussion. Second, I describe four leading theories about the nature of law and consider how central ideas in each theory can be seen to generate opposing ideas that lead in turn to opposing models of law. Third, I state briefly the affirmative case for thinking that the classical natural law view is correct. Fourth, I identify basic mistakes in current approaches to the question about the nature of law that help explain why modern positivism has overlooked the manner in which it leads logically to the classical natural law view. Finally, I add some brief remarks about why itmatters: what practical consequences follow from acknowledging that there are moral limits on what can count as law.