This volume focuses on different aspects of language development. The contributions are concerned with similarities and differences between first and second language acquisition, the acquisition of sentence structure and functional categories, cross-linguistic influence in bilingual first language acquisition as well as the relation between language acquisition, language contact and diachronic change. The recurrent topic of the volume is the link between linguistic variation and the limitation of structural variability in the framework of a well-defined theory of language. In this respect, the volume opens up new perspectives for future research.