"Research Findings:" Promoting children's science knowledge by adequate measures such as guided or free play is a cardinal goal of preschool. However, there is considerable variability in preschool teachers' instructional quality in block play, which might be associated with children's domain-specific science skills but also their mathematic and language achievement. To examine preschool teachers' instructional quality in a free block play episode we used a video-based assessment. We assessed children's knowledge in block play along with mathematics, language capacity, self-concept and cognitive skills. In order to obtain first insights into the association between teachers' practice and children's knowledge, we took a correlational approach. The sample consisted of N = 73 preschool teachers and N = 431 children. The results revealed considerable differences between preschool teachers' instructional quality. Overall instructional quality during block play as well as specific dimensions such as the use of spatial language, math language and cognitive activating scaffolding were positively associated with children's stability knowledge in block play. Moreover, preschool teachers' general language use and stimulation of communication as well as their sensitivity were positively associated with children's self-concept in block play. "Practice and policy:" Our study emphasizes the importance of preschool teachers' support for children's knowledge and self-concept and expands prior findings on early science learning.