In this study, prospective teachers' pedagogical content knowledge (PCK) of variables are examined by means of questionnaires answered by 184 prospective mathematics teachers from three different universities, and semi-structured interviews with 10 students. The respondents were from three different year groups. The data was analysed by putting responses into categories. The results suggest that the teacher candidates have different perceptions of the notion of the variable which are reflected in their pedagogical content knowledge in a complex way. The findings help our conceptualisation of PCK. This study suggests that also prospective teachers can possess PCK.