The purpose of this study was to examine the effects on four-year-olds' knowledge of mathematics by introducing professional development and center-based mathematics activities around four mathematical domains to early educators' teaching in Head Start programs. Because of the need to provide necessary mathematical experiences to young children to improve their early understanding and skills and provide the foundation for future success in mathematics, we provided the treatment group of early educators with professional development and center-based activities to promote four critical areas in mathematics. By randomly selecting Head Start centers to participate as the treatment group or control group, we were able to examine the effects of the professional development and set of activities on preschool children's knowledge over a six-month period. We found children in the treatment group were more fluent and flexible with number concepts, were better at solving contextual problems, and had better measurement and spatial abilities than children in the control group.