Metaethics, understood as a distinct branch of ethics, is often traced to G. E. Moore’s 1903 classic, Principia Ethica. Whereas normative ethics is concerned to answer first order moral questions about what is good and bad, right and wrong, metaethics is concerned to answer second order non-moral questions about the semantics, metaphysics, and epistemology of moral thought and discourse. Moore has continued to exert a powerful influence, and the sixteen essays here (most of them specially written for the volume) represent the most up-to-date work in metaethics after, and in some cases directly inspired by, the work of Moore.