Research indicates that U. S. teachers' mathematical knowledge continues to be weak, and there is an inherent difference between the mathematical knowledge needed to be an effective teacher and that needed by a mathematician. In this article, the author discusses pedagogical content knowledge. According to Schulman (1987), pedagogical content knowledge is the ability of the teacher to transform the content knowledge into forms which are "pedagogically powerful and yet adaptive to the variations in ability and background presented by the students." Furthermore, the author discusses the five components of the Fennema and Franke's (1992) teachers' knowledge model: (1) the knowledge of the content of mathematics; (2) knowledge of pedagogy; (3) knowledge of students' cognitions; (4) context specific knowledge; and (5) teachers' beliefs. Thus, the author states that the content of mathematics includes teachers' knowledge of the concepts, procedures, and problem-solving processes within the domain in which they teach, while pedagogical knowledge is teachers' knowledge of teaching procedures and learners' cognitions include knowledge of how students think and learn.