Studies in modality comprise a complex canon of functional, formal, sociological and diachronic analyses of language. The current understanding of how English language speakers use modality is unclear; while some research argues that core modal auxiliaries are in decline, they are reported as increasing elsewhere. A lack of contemporary and representative spoken language data has rendered it difficult to reconcile such differing perspectives. To address this issue, this article presents a diachronic study of modality using the Spoken BNC2014 and the spoken component of the BNC1994. We investigate the frequency of core modal auxiliaries, semi-modals, and lexical modality-indicating devices (MIDs), as well as the modal functions of the core modal auxiliaries, in informal spoken British English, between the 1990s and 2010s. The results of the analysis are manifold. We find that core modal auxiliaries appear to be in decline, while semi-modals and lexical MIDs appear relatively stable. However, on a form-by-form basis, there is significant evidence of both increases and decreases in the use of individual expressions within each modal set. As a result, this study problematises form-based studies of change, and illustrates the value and coherence that functional analyses of modality can afford future work.