Social–emotional competence is important for children's lifelong positive developmental outcomes. The dimensionality and psychometric properties of a widely used social–emotional assessment, the Ages & Stages Questionnaires: Social–Emotional Second Edition (ASQ:SE-2), were investigated in this study using item response theory models. A comparison was conducted between a unidimensional model, reflecting a compound construct—social–emotional competence, and a multidimensional model, based on the theoretical framework that posits that social competence and emotional competence are highly related but different constructs. The result indicated that the multidimensional structure presented a better fit for the ASQ:SE-2 sample across most intervals (i.e., 6, 12, 18, 24, 30, 36, 48, and 60 months). The psychometric properties estimated by the multidimensional model reflected congruence between the levels of item difficulty and the design of the ASQ:SE-2 and indicated a high quality of items based on the fit item statistics. However, the expected a posteriori/plausible value reliability estimates for several younger age intervals (i.e., 6, 12, 18, and 24 months) did not meet the 0.70 benchmark. The relationship between the 2 dimensions presented a moderate correlation during infancy (r = .43–.56) and a stronger relationship in older ages (r = .71–.83).