This chapter provides a review of the acquisition of the Welsh lexicon. Because Welsh-speaking children are growing up as both Welsh and English speakers, consideration of the acquisition of Welsh in relation to English allows a comprehensive picture of development. The chapter first explores what an examination of the number of lexical items a child knows in the two languages reveals, and this is followed by an examination of other factors central to lexical knowledge. These include children’s acquisition of the collection/unitiser system in Welsh, the acquisition of mutation and its ramifications for the acquisition of grammatical gender, and bilinguals’ processing of semantics in their two languages when these carve up the semantic space differently. Discussion includes ramifications for practitioners.