The theme of this volume was inspired by the theme of the VIIIth International Congress for the Study of Child Language which was held in San Sebastian, Spain, in July 1999. The chapters in this volume are based on papers that were presented at that meeting. They provide a snapshot of the current state of research on bilingual acquisition and reflect the diversity of issues, methodologies, and language combinations that can be found in contemporary work in the field. Topics include: the simultaneous acquisition of two first languages - early differentiation and subsequent development of grammars; the emergence of verbal morphology and the lead-lag pattern issue in bilingual acquisition; early language differentiation in bilingual infants; language differentiation in bilingual infants - evidence from babbling; and past tense verb forms, discourse context and input features in bilingual and monolingual acquisition of Basque and Spanish.