This study investigates beginning US elementary teachers’ competence for teaching mathematics and its development during teacher preparation and into the first 2 years of full-time teaching. Data are drawn from three longitudinal case studies and include the classroom video analysis survey, classroom observations and interviews about teachers’ instructional decisions, and whole-day shadowing. A multi-case study design was used to examine the processes of perception, interpretation, and decision making in participants’ comments on video clips of teaching episodes and in reflections about their own teaching. Findings support the central role of these processes in teacher competence and the generative power of reflections revolving around student thinking and tools, such as classroom discourse and visuals. Teachers’ communities also played an important role in teachers’ decision making. A model of teacher competence from a situated perspective is proposed and the classroom video assessment is discussed as a measure of teacher competence in context.