Through two studies, this work examined the applicability, interpretability, and construct validity of the Classroom Assessment Scoring System K-3 (CLASS) to measure quality of classroom interactions. In the first study, the CLASS was used in 332 classrooms to test three alternative models (in time order, the one-, three-factor, and two-factor models) to examine its factorial structure. The one-factor model showed worse fit than the other two models. The latent factors of the three-factor model were highly correlated. The bifactor model showed adequate fit. The aim of the second study was to investigate the construct validity of the CLASS. We used data collected from 31 classrooms to examine associations between factors extracted from the bifactor model with outcome variables in the domains of the student-teacher relationship, behavioral problems, and academic achievement. General- and domain-specific factors revealed different patterns of associations with child outcomes. The results are discussed relative to the Italian context.