Theoretical accounts of genocide and mass atrocity commonly embrace the thesis of norm transformation. This thesis holds, first, that individual and institutional participation in such crimes is at least partially explained by transformations in basic norms that structure social and political life. It holds, second, that preventing future occurrences of such crimes requires changing norms that currently govern the actions of particular individual and institutional actors. This paper clarifies, defends, and extends the thesis of norm transformation. It clarifies this thesis by providing a general account of the nature and dynamics of norms. It defends this thesis against charges of circularity and against the claim that norms are not, in fact, fundamental guides to action. Finally, it extends this thesis by arguing that changes in norms before, during, and after mass atrocities count among the considerations that ought to be included in assessments of legal and moral accountability for such crimes.