Research Findings: This article describes the development and evaluation of the Behavior and Emotion Expression Observation System (BEEOS), a direct observation tool to characterize preschoolers' social and emotion behaviors during semistructured activities in the classroom. The BEEOS was used to observe 148 Head Start preschoolers, and questionnaires were completed by teachers and parents to provide psychometric comparisons to the observational system. Findings support both the reliability and criterion-related validity of the behavior indicators and emotion expressions that make up the BEEOS. Preschoolers in the current sample engaged in expected patterns of social behavior and emotion expression. Furthermore, both constructs were related to teacher and parent reports of preschoolers' social behavior, externalizing and internalizing symptomatology, emotion regulation, and perceived emotion expressions. Practice or Policy: Findings suggest that the BEEOS can be used to supplement caregiver report and standardized assessments of preschoolers' social and emotional behaviors. The potential applications and implications of the BEEOS for research and education are discussed and include informing teaching planning and practices as well as the effectiveness of social-emotional interventions.