Although Lon Fuller’s importance and reputation among those who practice general jurisprudence remains contested, it is clear that he remains a major figure in developing the idea of the rule of law. Fuller may have been mistaken in believing that his desiderata of the internal morality of law were definitional of law, and in labeling these procedural desiderata a morality, but by providing a detailed account of eight different dimensions of what would now be thought of as the rule of law, he offered a substantial advance on the simpler accounts offered earlier by Dicey and others. And by positing a contingent and probabilistic relationship between compliance with his desiderata and substantive first order morality, Fuller offered what is best understood as a genuinely testable hypothesis about the rule of law and its value. This entry, prepared for forthcoming the Routledge Handbook on the Rule of Law (Michael Sevel ed.), seeks to support and explain the foregoing understanding of Fuller and his contributions.