This paper explores moral reasoning within the framework of contemporary cultural theory, in which moral functioning is action mediated by tools (such as socially available discourses) within a social and cultural context. This cultural model of a dialogic moral self challenges many of the assumptions inherent in the individualistic Kantian position that underlies much moral reasoning research. It provides a model for understanding cultural variation in ethical systems as well as the social context in which individual reasoning operates and develops. This framework derives from the developmental psychology of Vygotsky and from discursive social psychology, drawing on Foucault and Bakhtin. The core processes are discursive: dialectical and dialogic relationships at the intersections of three parts of a system comprising societal-cultural context, dyadic interaction and the individual agent. Key processes include the cultural and social construction of moral narratives and discourses that provide explanations and justifications within shared legitimation, comprehension and value. These provide resources that are drawn upon both in dialogue and in individual reasoning. Interpersonal interaction, a crucible and also a scaffold for both individual reasoning and for social change, involves argumentation, rhetoric and positioning.