This chapter proposes a new framework for the study of the categories of heresy and orthodoxy in medieval Islam. It engages with scholarship on these categories in the discipline of Islamic Studies and Religious Studies more broadly, arguing that orthodoxy and heresy should not be viewed solely through the prism of theology, which emphasises belief (doxa) and practice (praxis). Instead, it highlights the discursive formation of conceptions of orthodoxy and heresy. Heresy and orthodoxy are defined as shifting categories of denunciation and approval, which provides insights into the formation of orthodoxy as a contested process. This approach allows for a more detailed and complex typology of proto-Sunni movements in the eighth to eleventh centuries, admitting more variation than previous scholarship has allowed for.