In this study, we performed a fine grained analysis of writing by children with a specific language impairment (SLI) and examined the contribution of oral language, phonological short-term memory (STM), nonverbal ability, and word reading to three writing constructs (productivity, complexity and accuracy). Forty-six children with SLI were compared with 42 children matched for chronological age, receptive vocabulary (N = 46) and reading decoding (N = 46) on a measure of narrative writing. The SLI group performed worse on all measures compared to children of a similar chronological age. The SLI group produced a greater proportion of orthographic spelling errors than children with similar receptive vocabularies, but were comparable to children matched for reading decoding. The children with SLI showed specific difficulties in the omission of whole words (e.g. auxiliary verbs and subject nouns) and omissions of grammatical morphology (e.g. past tense--"ed") reflecting the difficulties shown in their oral language. Receptive grammar made a significant contribution to writing complexity and accuracy. Phonological fluency contributed to writing productivity, such as the production of diverse vocabulary, ideas and content and writing fluency. Phonological STM and word reading explained additional variance in writing accuracy over and above the SLI group's oral language skills.