Scholars of language development have long been challenged to understand the development of functional categories. Traditionally, it was assumed that children's language development initially relies on lexical elements, while functional elements become accessible only at later periods; and that it is lexical growth which bootstraps grammatical development. Over the last decades, however, a growing body of empirical research has come to contradict the traditional view. This new research involves a wide range of methodologies (e.g. discrimination, comprehension, production, neurophysiological) and a variety of languages, and extends the study of functional categories throughout development, beginning in infancy. In this article, the authors review a transformation that has occurred in the field. While lexical categories have long been assumed to be foundational to language acquisition, now functional categories have been revealed to play a foundational role. The article selects highlights of critical evidence emerging from various methods, languages and developmental periods, and articulates leading questions that now confront our field. Evidence suggests that language acquisition in the child, which begins long before first words, includes a continuous acquisition of functional categories. Functional categories provide a skeleton for sentence construction and a foundation for grammatical organization throughout acquisition.