This thesis investigates children’s mental representation of syntactic structure and how their acquisition and production of syntax is affected by lexical and semantic factors. It focuses on a frequent subject of language acquisition research: the passive. It is often claimed that English-speaking children acquire the passive relatively late in language development; previous studies have typically found unreliable comprehension and infrequent production of passives by children younger than five (eg, Fraser, Bellugi & Brown 1963). Other research has suggested that children’s early passives tend to involve variations from the full be-passive, for example, short passives (Horgan 1978) or get-passives (Harris & Flora 1982), but are usually restricted to a semantic core of agent-patient actional verbs (Maratsos, Fox, Becker & Chalkley 1985). Researchers have suggested therefore that children gradually form a passive representation based around these variations. However, there is evidence from studies that provide appropriate pragmatic contexts for passives (Crain, Thornton & Murasugi 2009) or that increase children’s exposure to passives (Whitehurst, Ironsmith & Goldfein 1974), which suggests children can produce, and so acquire, this structure at a younger age than previously assumed.