The purpose of this study was to empirically examine the dimensionality of language ability for young children (4-8 years) from prekindergarten to third grade (n = 915), theorizing that measures of vocabulary and grammar ability will represent a unitary trait across these ages, and to determine whether discourse skills represent an additional source of variance in language ability. Results demonstrated emergent dimensionality of language across development with distinct factors of vocabulary, grammar, and discourse skills by third grade, confirming that discourse skills are an important source of variance in children's language ability and represent an important additional dimension to be accounted for in studying growth in language skills over the course of childhood. [This paper was prepared by the authors who represent the Task Force of the Language and Reading Research Consortium (LARRC).]