This paper deals with competence and variation in bilingual children Following Baker (2006), an essential distinction is made between language ability and language use. After a general survey of the most important issues in the field, such as the separate development hypothesis, codemixing and transfer, a discussion of why and how child bilingualism sheds new light on the competence / performance distinction follows. It is argued that bilingual children can not be reduced to a questionof one linguistic (grammatical) competence, similar to that of monolinguals. On the contrary, the variations in performance should be considered as part of a diversity of competences which characterizes bilingual children and make it necessary toconsider them in their own right rather than as incomplete monolinguals. Some results from a new research project on simultaneous and successive bilingualism concludes the paper, showing that the age of onset of acquisition interacts withseveral factors, such as acquisition context, input, task demands and linguistic component (e.g morphology versus semantics), that would need to be better accounted for in the future.