The purpose of the current study is to describe the development and validation of a new measure of temperament, the Preschool Temperament Classification System (PTCS). The PTCS was developed as a typological measure that identifies children's temperament styles as undercontrolled, resilient, or overcontrolled. The PTCS is a time efficient structured interview assessing children's temperament as it is observed in the classroom environment. During the 20 minute interview, teachers classify all of the children in their class into one of the three temperament styles, rank order children within each temperament group, and assign an intensity rating to each child, indicating how well the child matches his/her assigned temperament group. Data were collected on 196 preschool-age children from 25 Head Start classrooms in an urban area. Teachers assessed children's temperament and peer play behaviors, and trained research assistants assessed children's school readiness. The PTCS classified children into temperament groups that showed expected relations to peer play competence and school readiness after controlling for age and gender, indicating good validity. Test-retest reliability was moderate to high. Overall the PTCS shows promise as a valid and reliable teacher measure of preschool children's temperament.