The present study examined the social-emotional development items assessed by kindergarten teachers in the Early Childhood Longitudinal Study-Birth Cohort to determine the optimal factor structure underlying the items as well as the reliability and validity of the resulting factors. This study identified an empirically derived factor structure for teacher-reported social development, investigated whether there was evidence of bias in teacher assessments of social-emotional constructs, examined factor invariance across demographic characteristics (i.e., race and ethnicity, sex, and poverty status), and examined the external validity of the derived factors by determining the extent to which they were associated with well-established measures of early childhood competencies. Findings suggested a 4-factor solution was optimal, consisting of (a) Interpersonal Skills, (b) Externalizing Behavior, (c) Approaches to Learning, and (d) Perspective Taking. Findings offer suggestive evidence of teacher biases in assessments and some, although not conclusive, support for the invariance of social-emotional dimension across demographic characteristics. Results provide a useful next step toward documenting reliable and valid social-emotional measures for use in early childhood research and challenges users of national datasets to think critically about the use of “scales” without a priori attention to important psychometric properties. (author abstract)