<p>Teachers’ professional noticing, a key teaching competency, enables them to address critical moments with tailored strategies, yet research on mathematics teachers’ noticing during instruction remains limited. This case study employs mobile eye-tracking technology and retrospective interviews to examine differences in noticing skills between an expert and a novice mathematics teacher in authentic classroom settings, including their perception, interpretation, and decision-making processes. Both teachers taught two eighth-grade lessons while wearing Tobii Pro Glasses 2, followed by video-based reflections on their noticing processes after teaching. Results show that the expert teacher distributed attention more evenly and identified a broader range of student needs, while the novice focused mainly on students who actively participated. Both teachers demonstrated descriptive, evaluative, and interpretive stances in interviews; however, the expert engaged in deeper reasoning by linking student errors to prior learning experiences, whereas the novice attributed student confusion to disengagement. Their distinct knowledge, beliefs, and goals further shaped their decision-making when addressing students’ cognitive misunderstandings.</p>