This study investigates topic units and suggests some practical applications to the teaching of composition to native speakers and second language learners. A topic unit is a structural unit beyond the sentence. It is composed of clauses and consists of two parts -- a head proposition and a set of comments. The head proposition consists of the first and sometimes second clause. It serves as topic and is joined to its comments by the "framing" function of a topic and by cohesive ties. The constituents, each consisting of one or a combination of clauses, are united among themselves on the basis of similarity. Because they are composed of clauses, topic units interact freely with sentences and with paragraphs to produce integrated texts and to offer means of stylistic variation within texts. (Author/JK)