We examined growth trajectories of writing and the relation of children's socio-economic status, and language and/or speech impairment to the growth trajectories. First grade children (N = 304) were assessed on their written composition in the fall, winter, and spring, and their vocabulary and literacy skills in the fall. Children's SES had a negative effect on writing quality and productivity. Children with language and/or speech impairment had lower scores than typically developing children in the quality and productivity of writing. Even after accounting for their vocabulary and literacy skills, students with language and/or speech impairment had lower scores in the quality and organization of writing. Growth rates in writing were not different as a function of children's SES and language/speech impairment status. Theoretical and practical implications are discussed. [This paper was published in "Elementary School Journal" (EJ1068973).]