Teachers' ratings of children's behavior are an essential tool of psychological research and practice. Previous research has reported levels of agreement between parents' and teachers' report of children's behavior, but relatively little attention has been paid to the agreement between teachers or between teachers and observers. In general, teacher-observer agreement has been found to be superior to parent-teacher agreement, but inferior to inter-teacher agreement. The present study included 170 children. Twenty-nine preschool teachers reported on the behavior of the children in their classroom, using Achenbach's Teacher Report Form. Research assistants coded children's behavior using videotapes of the preschool classrooms. Agreement between teacher reports was stronger than teacher-observer agreement. Standardization of teacher-report data within classrooms resulted in improved agreement, suggesting that reliability may be improved by focusing on report of children's behavior relative to other children in the classroom, rather than on an absolute scale. Inter-teacher agreement, and some types of teacher-observer agreement, showed significant variation across informant pairs. Finally, a teacher's perceptions of a child's behavior predicted aspects of the teacher's behavior toward the child, even after accounting for the child's behavior.