This study explores the nature of university students' experiences of understanding the threshold concept ‘function'. Fifteen teacher students taking an introductory course in mathematics were at the beginning of the course asked to reflect in writing on the meaning of ‘function'. Subsequent interviews explored in greater detail individual students' conceptual understandings of this concept. At the end of the course, students were again asked to reflect in writing on the meaning of function and then to elaborate further on those written reflections in a qualitative research interview. In a previous study of students' understandings of limit and integral an intentional analysis of the interview transcripts underscored the crucial interplay between understandings developed within an algorithmic and conceptual context respectively. The present study seeks to develop this contextual framework for analysing students' understanding in mathematics by linking the analysis of students' emerging understandings of function to ongoing research on threshold concepts in higher education and more broadly to research on students' experiences of understanding disciplinary ways of thinking and practising.