In Section 1 of this article, the author discusses the succession of models of adult writing that he and his colleagues have proposed from 1980 to the present. He notes the most important changes that differentiate earlier and later models and discusses reasons for the changes. In Section 2, he describes his recent efforts to model young children's expository writing. He proposes three models that constitute an elaboration of Bereiter and Scardamalia's knowledge-telling model. In Section 3, he describes three running computer programs that simulate the action of the models described in Section 2. (Contains 15 figures.)