In light of increasing demands on teachers and the need to develop teaching-related competences, this book examines the situation-specific skill of teacher noticing in pre-service and in-service secondary mathematics teachers. A video-based test instrument is used to measure teachers’ noticing skills in perception, interpretation, and decision-making from both general and mathematics pedagogical perspectives. The aim is to understand the structure and characteristics of teacher noticing across different groups, as well as the influences of teaching experience and opportunities to learn. Three quantitative studies are conducted: two cross-sectional studies with 457 participants, including master’s students, early career teachers, and experienced teachers, and one longitudinal study with 175 master’s students. The results support the conceptualization of teacher noticing as comprising three facets. They also reveal positive influences of teaching experience on the development of teacher noticing, with in-service teachers outperforming master’s students. However, experienced teachers perform similarly to early career teachers in general and worse in certain areas, suggesting saturation or forgetting effects. The longitudinal study finds that interpretation skills facilitate the development of perception and decision-making, emphasizing the knowledge-based nature of teacher noticing.