The purpose of this study is to describe teachers' knowledge about their students' algebraic thinking and to determine to what extent this knowledge essentially reflects students' ways of thinking. Three mathematics teachers and their 49 ninth grade students were selected as participants. In this study, students' algebraic thinking was determined through a generalization activity and then the teachers' knowledge and expectations were investigated. The data were comprised of students' worksheets and the interviews with the teachers. The results indicated that there was a considerable difference between students' actual ways of algebraic thinking and the teacher's expectations. However, it was also revealed that the teachers were able to better understand students' ways of thinking after examining their students' worksheets carefully.