The ways in which preschoolers use the word "but" were studied. It was found that the eight preschoolers, who ranged in age from 3;9 to 5;5, were able to use "but" to express a number of different types of adversative relationships. "But" introduced clauses containing information that: (1) contrasted with shared knowledge about the usual state of affairs, (2) described a condition under which the situation described in the preceding clauses did not hold, (3) denied an inference that might be drawn on the basis of the preceding statement, (4) explicitly contradicted a prior statement, and (5) introduced a temporal repair. The cognitive prerequisites that could be assumed to underlie and motivate each of these types of "but" statements also are considered. Statements contained in categories one and two indicate knowledge of optional pathways in the occurrence of an event, while statements contained in category three indicate at least an emerging ability to recognize the implications of one's own statements, and possibly to take the listener's perspective and make inferences about her inferences. Statements in categories four and five indicate knowledge of and adjustment to discourse conventions. The subjects' productions are divided into categories and discussed on the basis of whether the contrast signaled by "but" was directed toward the text of the statement, or toward the discourse process per se. It is suggested that the analyses indicate a somewhat surprising level of sophistication in children's understanding of discourse conventions and their ability to adjust their speech in accord with such conventions. (SW)