This new and pathbreaking study provides the first ever comprehensive account of how children acquire complex sentences. Drawing on observational data, Holger Diessel investigates the spontaneous speech of English-speaking children aged between two and five, examining the acquisition of infinitival and participial complement clauses, finite complement clauses, finite and non-finite relative clauses, and co-ordinate clauses. His investigation shows that simple, non-embedded sentences gradually evolve into biclausal constructions, and that two different developmental pathways can be distinguished: complement and relative clauses evolve from simple sentences that are expanded to multiple clause constructions; and adverbial and co-ordinate clauses develop from simple sentences that are integrated into a specific biclausal unit. He argues that the acquisition process is determined by a variety of factors: the frequency of the various complex sentences in the ambient language, the complexity of the emerging constructions, the communicative functions of complex sentences, and the child's social-cognitive development.