In order to use concept maps in physics classes effectively, teachers' knowledge and ideas about concept mapping are as important as the physics knowledge used in mapping. For this reason, we aimed to examine pre-service physics teachers' knowledge on concept mapping, their ideas about the implementation of concept mapping in physics classes, the hidden elements influencing their ideas, and the relations between knowledge and ideas, qualitatively. The participants of this study were eight pre-service physics teachers enrolled in the physics education department at two state universities. The results of the interviews conducted with the participants and their artifacts revealed that although pre-service physics teachers had basic knowledge about concept mapping, they had some negative ideas about implementation in physics classes. Furthermore, language, limitation in assessment, limitation in expressions, and teacher's knowledge were identified as the sources of pre-service physics teachers' negative ideas.