This chapter examines arguments about the morality of lying and the limits of our knowledge about the rightness and wrongness of lying. It defends a version of the golden rule and several other rationality requirements as negative tests for moral judgments. It argues that certain moral views can be discredited or shown to be false, but that, for many moral issues, including lying, there is a range of reasonable but incompatible views. These reasonable views include both act-utilitarianism and Ross’s theory, but not egoism or absolutism. The author conjectures that the range of reasonable views excludes all views that permit lying in cases in which both act-utilitarianism and Ross’s theory prohibit it—lying is wrong in all cases in which both theories forbid it. The chapter concludes by addressing the moral relevance of the distinction between lying and deception.