In countries with a social pedagogic tradition for early childhood education, mathematical learning typically takes place in play-based situations. Preschool teachers' ability to recognize mathematical content in children's play is therefore an important prerequisite for educational quality. The present study examines how this ability relates to other aspects of preschool teachers' professional competencies. Findings from regression analysis indicate that mathematical content knowledge (CK) predicts teachers' sensitivity to mathematical content. However, further analyses reveal that this association is mediated by preschool teachers' self-efficacy beliefs. (author abstract)