The PSOM was developed through an iterative process that involved developing a preliminary rubric, watching and coding videos, discussing the videos in detail, and revising the rubric accordingly, carried out by a research team that represented expertise in preschool science and observational measurement. This process was continuously informed by the team’s review of relevant literature and consideration of classroom observation systems related to science and classroom processes more generally. Through this process, we developed the PSOM to capture the content of science instruction as well as the quality of the instruction during structured whole- or small-group lessons. The content items captured the specific concepts being taught in a given lesson as well as opportunities to engage with science tools and science processes. Items were based on material taught in existing pre-K science curricula (including the one under study) as well as on a review of state pre-K science standards and NGSS practices and disciplinary core ideas (National Research Council, 2012). We structured the quality items to form three conceptual domains (Engage, Explore, and Consolidate) that reflect our understanding of the cognitive science of learning as well as to echo the widely used constructivist instructional framework of the five E’s (Engage, Explore, Explain, Elaborate, Evaluate; Yoon & Onchwari, 2006). In the PSOM, Engage items capture how teachers introduce a lesson, Explore items capture how opportunities for learning transpire, and Consolidate items capture how teachers connect and restate information during and at the end of the lesson. (author abstract)