Seven Finnish mentor mathematics teachers were interviewed about their views regarding the teacher knowledge required for teaching mathematics. The results of the interviews revealed not only the teachers' spontaneous views of the knowledge base needed for effective mathematics teaching but also their views of the particular types of teacher knowledge required for teaching mathematics that have been emphasized in the research literature. When the teachers freely described their views, issues related to content knowledge and the requisite knowledge for teaching mathematics were emphasized. The teachers valued most of the knowledge types available, even if only a few of them had highlighted them earlier in the interview. Some types, however, remained ambiguous from the teachers' perspective. Our findings suggest that even if the mentor teachers are considered experts in mathematics teaching, they are not necessarily able to conceptualize their view of the knowledge required for mathematics teaching, or they may have their own views of the necessary types of knowledge. This in turn may impede student teachers in connecting their educational studies with mathematics studies during student teaching. In addition, it was found that mentor teachers frequently hold personal views about the importance of the types of mathematics teacher knowledge that may be valuable for student teachers when they are reflecting their own view of the requisite knowledge for teaching and the type of a teacher they want to become.