This is a comprehensive study of the passage from first words to grammar in a sample of children large enough to permit systematic analysis of individual differences in style and rate of development. The authors provide a large body of information about first words and early grammatical development in qualitative and quantitative patterns that are useful not only for researchers in the field, but for speech/language pathologists and early childhood educators interested in the assessment of early language. They also address one of the most controversial theoretical issues in modern linguistics and psycholinguistics: the problem of modularity, with individual differences suggesting that components of language can come apart in early stages, developing at different rates in different children. But these differences appear to cut across the supposed boundaries between grammatical and lexical development, suggesting that the same mechanisms are responsible for both. The results support a unified functionalist approach to language development, and have implications for the way we think about the structure and breakdown of language under normal and abnormal conditions.