This explanatory sequential mixed methods study investigated how a professional learning system involving an interplay of teachers' own classroom video, analysis tools and coaching helped teachers notice students' thinking. The system comprised teachers scrutinizing video evidence of their students' ideas, using software tools to analyze the particulars of student thinking, and engaging in video studies with a coach to sharpen their emerging insights. Thirteen elementary grade science teachers participated in a professional learning program. They videotaped their science discussions and used a video analysis software to study their videos with a program coach. Transcripts of teachers' baseline and post-program video studies were analyzed quantitatively. For three of those teachers, transcripts of their video studies during the program were analyzed qualitatively. All teachers increasingly attended to and interpreted students' thinking and focused on specific student ideas. They studied noteworthy events from the video and adopted a particular "lens" to scrutinize their students' thinking. Teachers drew on their knowledge of the curriculum and students and feedback from their coach. The paper elaborates key findings and implications in light of the relevant literature.