It is becoming widely recognised that teachers' content knowledge has an important influence on their pedagogical content knowledge, and hence on the learning of students. In secondary schools, function is one of the fundamental concepts of mathematics. This paper considers the understanding of function exhibited by a group of teacher trainees in response to various representational presentations. The results show that there is a wide range of differing perspectives on what constitutes a function, and that these perspectives are often representation-dependant, with a strong emphasis on graphs. (Contains 6 figures and 1 table.) [For complete proceedings, see ED500860.]