Thoughtful implementation of the Common Core State Standards for Mathematics (CCSSM) (CCSSI 2010) presents an opportunity for increased emphasis on the development of mathematical understanding among students. Granted, ascertaining the mathematical understanding of an individual student is highly complex work and often exceedingly difficult. Although textbooks may provide practitioners with considerable overarching instructional guidance, to complete the picture, mathematics teachers must often focus on individual children. In some instances, they might consider verbal explanations or work samples to gain insight into one's thinking; however, quite often these avenues do not provide a complete or accurate portrayal of a student's understanding. Indeed, a student may unwittingly offer an explanation that differs from her actual strategy (CCSSI 2010). Similarly, work samples may feature deceptive or insufficient details to truly gauge the student's thinking. In these instances, a systematic approach to appraise the mathematical moment is required to fully appreciate the student's true understanding and then respond with effective instructional tactics. This article begins by describing the three interrelated phases of the "Professional Noticing of Children's Mathematical Thinking" (Jacobs, Lamb, and Philipp 2010) framework for teachers to better understand and act on their students' mathematical conceptions and practices. The second half of the article addresses how to put the framework into practice through the progression of the three phases (Attending, Interpreting, and Deciding).