The concept of proof has an important place in mathematics education. Mathematics lesson curricula although it provides teachers and students with rich opportunities and experiences about the nature and role of proof, teachers' response to their students in this context largely depends on teachers' knowledge of the concept of proof. In this study, the opinions of 35 teachers, 24 primary and 11 secondary mathematics teachers, working in schools, on the concept of proof were examined. The study was conducted based on case study, which is one of the qualitative research designs. The data of the study were collected through a semi-structured interview form developed by the researcher. The collected data were analyzed by content analysis method. According to the results of the research, it was determined that the teachers mostly associated the purpose of proof with the category of "Persistence". Many of the teachers emphasized that the use of proof in the teaching process allows students to learn the information without memorizing, and thus the learned information is permanent in the minds of the students. In addition, the results of the research showed that teachers generally have limited views on the concept of proof. This study was carried out with a limited number of teachers. Similar studies can be carried out by increasing the number of participants. Studies can be conducted on teachers' levels of proof, the approaches, methods, or strategies they use in the process of proving. By making observations in practice environments for the use of proof, the frequency of teachers' use of proof in the classroom can be determined more clearly.