This report is concerned with the sources of language development in the child. We have found a substantial relation between naturally occurring variations in children's language environments and their language skills, both at the earliest stages of language development and later, at 5 and 6 years of age. Our studies show that children's language development is related to the speech they hear at home and the speech they hear at school. Although much more work remains to be done, it is clear that differences in language input within the normal range are causally related to the growth of children's vocabularies and their syntactic skills.