This study examined the internal structure of the Classroom Assessment Scoring System (CLASS; K-3 version). The original CLASS K-3 model (Pianta, La Paro, & Hamre, 2008) and 5 alternative models were tested using confirmatory factor analysis with a sample of first- and second-grade classrooms (N = 141). Findings indicated that a slightly modified version of the original CLASS K-3 3-factor model best fit the current data. Although stable findings emerged across the current and previous studies, particularly in relation to the presence of 3 latent domains, there is also some variability across structures at different grade levels with regard to the bifactor and 3-factor models. [This paper was published in "School Psychology Quarterly" (EJ1143193).]