Research Findings: The purpose of this study was to test a theoretical measurement model representing four proposed dimensions of the classroom ecology in pre-K to third-grade classrooms. The four proposed dimensions of Classroom Composition, Peer Network and Norms, Teacher Practices, and Student Experiences were evaluated using data collected in 182 classrooms and 2,662 students in two districts spanning rural, suburban, and urban settings. Overall, the theoretical measurement model supported the multi-dimensional nature of the classroom ecology across the early primary grades, although specific characteristics and relations within the four dimensions may vary somewhat in pre-kindergarten settings compared to kindergarten through third grade. Practice or Policy: The overarching goal is to advance research that conceptualizes the classroom ecology more broadly to reflect both the academic and peer environment. The study is important for advancing understanding of salient characteristics of the classroom ecology that may foster learning and achievement. [For the corresponding grantee submission, see ED619913.]