Erikson Institute, partnering with Head Start programs of the Children and Youth Services Division of the City of Chicago, will examine the relationship between teachers' pedagogical content knowledge for preschool math and both math-related teaching practices and gains in young children's mathematics achievement. The study has three main objectives: (a) to determine what types of and how much preschool math pedagogical content knowledge teachers possess; (b) to examine whether preschool math pedagogical content knowledge predicts math-related teaching practices; and (c) to compare preschool math pedagogical content knowledge and math-related teaching practices as predictors of children's gains in math achievement. Participants in the study will include 60 Head Start teachers and 360 four-year-old children. A new teacher interview based on literature review and review by experts will be used to evaluate teachers' preschool math pedagogical content knowledge. Teacher speech samples, classroom observations and teacher background surveys will also be employed. Children's mathematical gains will be assessed using the Test of Early Mathematics Ability, 3rd Ed. (TEMA-3). Results from the study will be used to develop plans for teacher professional development to improve the mathematical skills of children in Head Start.