This study aimed at evaluating preschool teachers' knowledge of early mathematical development. To this end, the validity and reliability of the 'Knowledge of Mathematical Development (KMD) Survey,' were determined by adapting it to Turkish culture, and using this tool, the participants' KMD was examined with different variables. The research study was carried out with preschool teachers working at public and private preschools in the city of Giresun, Turkey. The data on preschool teachers' knowledge of early mathematical development were collected through the KMD survey. The findings of this study indicate that the Turkish version of the KMD survey was determined to be a valid and reliable measurement instrument. KMD scores varied significantly, depending on the age of teachers and teachers' duration of service.