Prevalence estimates suggest that up to 20% of students in schools experience significant internalizing behaviors that impact behavioral, social, or academic functioning. School-based interventions have great potential to promote student mental health; however, validated and feasible brief assessments are needed to progress monitor students' responses to these supports. The purpose of the current study was twofold: to (a) develop and validate teacher-completed brief rating scales for progress monitoring internalizing concerns in elementary-age students and (b) determine the reliability of the resultant measures. First, item content was generated and subjected to evaluation by two panels of school-based consumers and researchers. Within the second phase of development, exploratory and confirmatory factor analyses were used to reduce the initial number of items and ensure that the items were indicators of one latent factor. Teachers in Grades K-3 (N = 307) each completed ratings for one randomly selected student in their classroom. Results of factor analysis for each scale indicated one-factor solutions for the 4-item Anxious/Depressed ([omega] = 0.88) and 4-item Socially Withdrawn ([omega] = 0.87) scales. [For the corresponding grantee submission, see ED614791.]