AbstractChildren acquire a mature language system and sometimes this system differs from that of their parents. This is a significant part of language change and understanding acquisition is key to understanding this kind of change in people's internal grammars. I outline one approach to language acquisition, based on children finding cues expressed in the input they are exposed to. This enables us to understand historical change in grammars: change in external language sometimes triggers a new internal grammar as cues come to be expressed differently. Work on language variation, acquisition, and change converges, and these three areas are mutually dependent; empirical work in one area may enrich understanding more generally, opening the way to new kinds of empirical work. Seen this way, language is a complex system and language change can be treated productively in the context of complexity science. Copyright © 2010 John Wiley & Sons, Ltd.This article is categorized under: Linguistics > Language Acquisition