Research has established the importance of play-based experiences for young children's development and learning. Yet, facing the mounting pressure of accountability and standardized testing, some educators have turned to didactic methods of instruction. Guided play (Weisberg et al., 2016) provides a middle ground between play and instruction for teachers to take a participatory role in children's playful learning. However, teachers have struggled to enter into children's play in ways that support learning. The purpose of this study was to identify and describe ways teachers actively engaged in supporting children's STEM learning through play. Using qualitative analysis methods, I analyzed video recordings of guided play sessions in Head Start preschool classrooms to describe children's and teachers' participation in spatial reasoning activities. Children participated through "exploring materials," "announcing activities," "repeating ideas," and "pursuing challenges." Teachers participated through six lines of action, four of which were described by Bjorklund et al. (2018): "structuring the material environment," "confirming direction of interest," "providing strategies," "situating known concepts," "challenging concept meaning," and "creating novelty." Analysis of two case episodes provided insights into co-participation in play, including the role of intersubjectivity in guided play activities. Findings from this study provide practitioners with more explicit examples of teaching approaches that support children's STEM learning during play. This study also provides theoretical insight into the intricacies of teacher and child co-participation during guided play, including the role of playfulness in teaching and learning. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]