We describe the process of relationship-based intervention and associated changes in children and teachers' relationships and behaviors within center-based child care and examine the circumstances under which changes were most likely to occur. The relationship-based intervention had three components: research partners in each participating classroom, focus groups, and feedback sessions. Twenty-eight teachers and seventy children from 10 programs participated in the research. Approximately half of the children and one-third of the teachers were Latino, and almost half of the teachers and one-third of the children African-American. We used five distinct procedures: (1) participant observation; (2) naturalistic observations using a time sampling procedure; (3) global ratings; (4) clinical interviews and (5) case studies. There were significant main effects for change over time in children's attachment security, complexity of play with peers, and their experiences of teacher responsive involvement. Teachers with BA degrees were more engaged in the intervention than teachers with less formal education. When we held BA degrees constant, children who had teachers engaged in the intervention experienced the greatest changes in classroom climate, teacher responsivity, and in their relationships with the teacher.