Welsh grammar is characterised by an interesting set of morphosyntactic structures. Unique features within these structures distinguish Welsh – along with Irish and Scottish Gaelic – from other Indo-European varieties, and these differences offer a novel lens through which we can explore how language is learned. How children acquire the structures of Welsh, and how these structures are used by adults, has been the focus of a growing body of research over the past few years. The results of these studies have helped shape our understanding of the linguistic profiles of different types of bilingual Welsh-English speakers, in terms of their rate and pattern(s) of learning, and have highlighted some of the key factors influencing potential and achieved linguistic outcomes when learning within a minoritised bilingual context, contributing new and important insights into the various theoretical debates in the field. In this chapter, we outline how various morphosyntactic structures work in Welsh, and provide an overview of what is known from the current literature about L1 and L2 acquisition of Welsh morphosyntax, as spoken by both typically and atypically developing bilinguals. The different types of methodologies that have been applied to the study of Welsh grammar with adults and children will be discussed throughout, and suggestions for future studies presented at the end.