Early childhood education emphasizes the need of providing high quality early childhood mathematics programs for preschool children. However, there is little research that examines the importance of preschool children’s mathematical knowledge development and teachers’ beliefs about how to teach mathematics to young children. The purposes of this study were to investigate pre-service and in-service preschool teachers’ knowledge of children’s mathematical development and their beliefs about teaching mathematics in the preschool classroom and also to determine how experience differentiates the two groups. This research employed a non-experimental research design with convenient sampling. Ninety-eight pre-service teachers and seventy-seven in-service preschool teachers participated in the research. The Knowledge of Mathematical Development survey (KMD) and the Beliefs survey were used to investigate possible differences between pre-service and in-service preschool teachers’ knowledge of children’s mathematical development and between their beliefs about teaching mathematics. The findings of this study indicate a statistically significant difference between pre-service teachers and in-service preschool teachers in relation to their knowledge of mathematical development. This finding shows that pre-service teachers’ knowledge of children’s mathematical development is somewhat limited; most pre-service teachers have difficulty identifying the process of preschool children’s development of mathematics skills. A second finding reveals a statistically significant difference …