The author takes up three metaphysical conceptions of morality realism, projectivism, constructivism - and the account of justification or reason that makes these pictures possible. It is argued that the right meta-ethical conception should be the one that entails the most plausible conception of reason-giving, rather than by any other consideration. Realism and projectivism, when understood in ways consistent with their fundamental commitments, generate unsatisfactory models of justification; constructivism alone does not. The author also argues for a particular interpretation of how "objective moral obligation" is to be understood within constructivism.