This study examined how individuals and peers process scientific information that contradicts what they believe and assessed the contribution of this activity to conceptual change. Participants included 54 students in Grade 9 and 54 students in Grade 12, who were randomly assigned to four conditions: (a) individual conflict, (b) peer conflict, (c) individual assimilation, and (d) peer assimilation. Depending on the condition, students were asked to think aloud or discuss with their peers eight scientifically valid statements, which were presented in an order that either maximized or minimized the conflict between new information and existing beliefs. Pretest and posttest measures of prior knowledge and conceptual change were obtained, and student verbalizations were tape-recorded and coded for five levels of knowledgeprocessing activity. Two major approaches were identified from this analysis: direct assimilation, which involved fitting new information with what was already known, and knowledge building, which involved treating new information as something problematic that needed to be explained. A path analysis indicated that the level of knowledge-processing activity exerted a direct effect on conceptual change and that this activity mediated the effect of conflict. Knowledge building as a mediator of conflict in conceptual change helps to explicate previous equivocal research findings and highlights the importance of students' constructive activity in learning. Requests for reprints should be sent to Jud Burtis, Ontario Institute for Studies in Education of the