In the past two decades there has been an increased effort to understand the depth to which mathematics teachers must know their subject to teach it effectively. Researchers have termed this type of knowledge mathematical knowledge for teaching (MKT). Even though recent studies have focused on MKT, the current literature on the subject indicates that this area remains underdeveloped. In an attempt to further refine our conception of MKT this study looked at MKT in Japan. In this thesis I explored and categorized the MKT of three experienced Japanese cooperating teachers (CTs) by looking at the content of their conversations with three Japanese student teachers (STs). I separated the MKT mentioned in these conversations into three categories: knowledge about the students' mathematical knowledge, knowledge about mathematics, and knowledge about school mathematics. I also discussed various implications of this work on the field of MKT.