The aim of this study is to identify the mistakes made by ninth grade students in the subject of functions and reveal the reasons for these mistakes. In line with this purpose, the case study method, which is among the qualitative research methods, was used in the study. The study group of the research was composed of 52 Anatolian ninth grade students and four mathematics teachers. Eight open-ended questions were used as the data collection tool. Non-structured interviews were conducted in order to reveal the reasons for the mistakes. The content analysis technique was utilized in analyzing the data. When the obtained findings were examined, it was observed that the majority of the students experienced problems in meaningfully perceiving and expressing the functions. When the data obtained from the interviews was examined, it was observed that the mistakes made by the students resulted from such reasons as lack of conceptual knowledge, memorization-based learning and the fact that they were not ready for the subject.