The Mathematical Development Beliefs Survey was developed to measure early childhood teachers’ beliefs about mathematics teaching and learning in the preschool classroom. This instrument was designed to measure beliefs concerning (a) age-appropriateness of mathematics instruction, (b) classroom locus of generation of mathematical knowledge (teacher vs child), (c) mathematical development as a primary goal of preschool education, and (d) confidence level in providing mathematics instruction. The reliability and validity of the instrument was examined through multiple phases of development, including two pilot studies and a final study with 346 pre- and in-service preschool teachers across three states. Methods included cognitive interviews with participants, literature reviews, and interviews with experts in the field, as well as statistical procedures such as analyses of variance between well-defined groups of in- and preservice teachers, correlations between measures of knowledge and beliefs, and confirmatory factor analysis. Reliability of the instrument was examined through the use of Cronbach’s alpha and item-total correlations. These statistical procedures provided very good to excellent support for both validity and reliability. Potential applications include informing development and evaluation of early childhood education teacher preparation programs and professional development interventions.