This article describes a type of professional development focused on students' thinking that occurs in middle school mathematics classrooms. This type of professional development is called "Classroom Embedded" and occurs over one or two days in a classroom with middle school students and their teacher. Teachers actively participate in the planning and implementation of problems designed to elicit and extend students' understanding of contextualized problem situations. Similar to Lesson Study, teachers observe the lesson and evaluate student responses. The emphasis is on assessment of students' mathematical understandings and making their thinking explicit through sharing of strategies and connections among key mathematical concepts and procedures. Aspects of both the common core content standards and the eight standards for mathematical practice are highlighted throughout the article.