This Element defends and clarifies the thesis that the legality of a system of rules depends on its moral features. Positivists who deny this dependence struggle to explain: (1) the traditional classification of moral norms as a form of a priori law; (2) judicial reliance on moral norms in legal discovery; (3) persistent theoretical disagreement about intra-systemic, law-determining facts; (4) why radically arbitrary or immoral schemes of social organization represent borderline cases of law; and (5) why law, like other artifacts, can be evaluated in a kind-relative sense (“as law”). Meanwhile, traditional versions of non-positivism overstate the dependence going further than the desiderata warrant. A moderate theory is formulated: law is an artifact whose existence depends on adequately performing an essentially normative function. The theory's justification lies in its explanatory power: a comparison with other “value-driven” artifacts, such as artworks, proves vital for understanding legal language, reasoning, and practice.