Book excerpt: This book is part of an ongoing and exciting scientific quest for uncovering the mechanisms that guide the acquisition and use of language in the human species. It focuses on a key mystery - children's acquisition of syntax, which is a critical component in all languages. The book investigates when and how some core syntactic constituents become accessible to the child, in particular, Functional Categories - the grammatical categories that are realized in language by a set of function words (e.g., the, and) and verb inflections. A comprehensive review on functional categories is followed by new empirical findings, which reveal that already around one year of age, infants incorporate syntactic information regarding function words in sentence processing. This grammatical sensitivity demonstrates that content categories (e.g., nouns, verbs) and functional categories are developing in tandem during early periods in language acquisition. The book provides new information about the linguistic development of children and is thus intended for both students and researchers in the cognitive sciences, as well as for a general audience, including health educators, parents, and teachers.