This paper argues that Habermas's conception of the rationality of moral and legal discussions has import for argumentation theorists interested in the rationality of public deliberations in politics and law. I begin with a survey of Haber mas's discourse theory and his criteria of rationality for moral and legal discourse. I then explain why, in his view, the forms of rational discourse in morality and law complement each other. My aim is to show how Habermas's account of this complementary relationship opens up fruitful perspectives for argumentation theory. Specifically, his thought can stimulate research regarding, on the one hand, the ways in which legal procedures provide for presumptively rational resolutions of moral disputes and, on the other hand, the applicability of ideal argumentation-theoretic models to the legal field. I conclude with a proposal for integrating Habermas's ideas in a research program for legal argumentation.