Background: The importance of diagnostic and scaffolding activities for early science learning has been shown consistently. However, preschool teachers scarcely engage in them. We developed an instrument to assess preschool teachers' willingness to engage in diagnostic and scaffolding activities in science learning situations and examined its relation with teachers' knowledge, beliefs and practice. Aims: We validate an instrument to assess willingness to engage in scaffolding and diagnostic activities and study the interplay between willingness, learning beliefs, content knowledge (CK) and pedagogical content knowledge (PCK) in the context of science learning, particularly block play. Sample(s): A total of N = 151 preschool teachers from 41 kindergartens in Germany participated in our study. Methods: Preschool teachers completed a questionnaire, which took approximately 1 hour of time. We drew a subsample of N = 73 teachers and observed their practice during a 30 min block play episode. Results: With our instrument, we were able to distinguish between preschool teachers' willingness to diagnose and to scaffold. Preschool teachers' co-constructivist beliefs and PCK predicted willingness to engage in diagnosing, PCK also predicted willingness to engage in scaffolding. Associations between learning beliefs and practice were inconsistent. Conclusions: Our study highlights aspects of the association between preschool teachers' PCK and their willingness to engage in diagnosing and scaffolding. However, we found inconsistencies between preschool teachers' beliefs and practice, which call for further clarification.