Noting that children's early mathematics experience is a significant factor in their future mathematics outcomes and attitudes, this paper discusses the importance of developmentally appropriate practices in mathematics instruction for young children and explores how teachers' pedagogical content knowledge determines their teaching practices in mathematics. The paper explains that pedagogical content knowledge refers to the integration of pedagogical, or teaching, knowledge and content knowledge; it refers to the way teachers transfer their subject matter knowledge into an understanding of particular topics, problems, or issues in order to present information to diverse learners' varying abilities and interests. The paper discusses four elements of pedagogical content knowledge of mathematics in early childhood settings: (1) teachers' understanding of mathematics content; (2) teachers' understanding of children's understanding of mathematics content; (3) teachers' understanding of children's mathematics problem-solving process; and (4) teachers' understanding of the organization of the mathematics environment based on children's understanding of mathematics content and the process of children's understanding of mathematics. (Contains 19 references.) (EV)