Mathematics educators are constantly searching for new and innovative approaches to teaching with the goal of improving student learning outcomes. This phenomenological study aimed to describe the lived experiences of mathematics teachers in implementing the Claim-Evidence-Reasoning (CER) Approach. The study was participated by 13 mathematics teachers, and data were gathered through in-depth interviews and a focus group discussion. Thematic analysis was used to interpret the data guided by Colaizzi’s method. Findings revealed that participants experienced struggles in instruction and assessment, adapted (...) growth mindset and employed support systems, had milestones in the implementation of the CER Approach, and were open to continuous personal and professional development. Furthermore, the teachers’ resiliency and adaptability in the process and the school’s support helped overcome the issues encountered in implementing the CER Approach. The findings imply that the use of CER Approach have the potential to improve the reasoning skills of students in mathematics. Further, it is recommended that mathematics teachers continue to utilize the approach and that educational institutions should provide proper training to teachers through In-Service Training (INSET). (shrink)