Research Findings: The aim of this study was to investigate U.S. preschool teachers' math teaching knowledge in a specific content domain: counting and numbers. One hundred in-service and pre-service teachers participated in the study; they completed a questionnaire that is composed of learning scenarios and scenario-based math teaching questions. A mixed qualitative and quantitative method was applied in analyzing the data. Results show that the majority of U.S. preschool teachers failed to identify the concept of count-out that is embedded in a math game, the one-to-one correspondence error was the most identified counting error comparing to the other errors such as the cardinality principle error, and the knowledge of content and teaching appears to be more satisfactory than specialized content knowledge or knowledge of content and students. Practice or Policy: The study serves to fill some gap in addressing a pressing need in the field that is to identify and measure the unique body of preschool teachers' math teaching knowledge. The study also justifies that preschool teachers need intensive training such as a coursework that wholly focuses on math teaching in order for their math teaching knowledge to be more refined and teaching to be more knowledge-informed.