Background: Deficient vocabulary is a frequently reported symptom of developmental language impairment, but the nature of the deficit and its developmental course are not well documented. Aims: To describe the nature of the deficit in terms of breadth and depth of vocabulary knowledge and to determine whether the nature and the extent of the deficit change over the school years. Methods & Procedures: A total of 25 681 oral definitions produced by 177 children with developmental language impairment (LI) and 325 grade-mates with normally developing language (ND) in grades 2, 4, 8 and 10 were taken from an existing longitudinal database. We analysed these for breadth by counting the number of words defined correctly and for depth by determining the amount of information in each correct definition. Via a linear mixed model, we determined whether breadth and depth varied with language diagnosis independent of non-verbal IQ, mothers' education level, race, gender, income and (for depth only) word. Outcomes & Results: Children with LI scored significantly lower than children with ND on breadth and depth of vocabulary knowledge in all grades. The extent of the deficit did not vary significantly across grades. Language diagnosis was an independent predictor of breadth and depth and as strong a predictor as maternal education. For the LI group, growth in depth relative to breadth was slower than for the ND group. Conclusions & Implications: Compared with their grade-mates, children with LI have fewer words in their vocabularies and they have shallower knowledge of the words that are in their vocabularies. This deficit persists over developmental time. (Contains 5 tables and 3 figures.)