Classroom observation and teacher supervision is an important part of the life of schools. From the teacher's point of view it can be an opportunity for one's principal to get to know one's work and provide opportunities for continued learning; or it can be an ordeal. From the principal's point of view it can be an opportunity to understand what is going on in mathematics classrooms in his school and to offer help to teachers; or it can be a perfunctory process of filling out a checklist. This article reports the results of a study conducted to determine the role principals' leadership content knowledge (LCK) plays in their observations of mathematics classrooms and conferences with teachers. The researchers were particularly interested in how different LCK "profiles"--particular combinations of principals' mathematics knowledge for teaching and their beliefs about mathematics learning and teaching--affected principals' goals for mathematics instruction and their approaches to the supervision of teachers. Data from case studies of elementary principals at work in their schools show that principals with different LCK profiles use very different criteria when evaluating mathematics classes. Their LCK for mathematics affects what they notice and value when observing math classes, what they talk about with teachers during post-observation conferences; and how they interact with teachers during post-observation discussions. Principals with different LCK profiles also provide very different kinds of support for teachers.