Based on attachment theory, two aims were addressed. First, we tested whether close teacher-child relationships may buffer children who are less securely attached to their mothers against negative outcomes, such as aggressive behavior. Second, our study evaluated whether teacher sensitivity may protect less securely attached children against forming less close relationships with their teachers. In a sample of 127 children, mother-child attachment was observed in preschool. In kindergarten, teacher sensitivity was observed and teacher-child closeness and child aggressive behavior were rated by the teacher. Results of multilevel hierarchical regression analyses first showed that with high teacher-child closeness, less securely attached children are no longer at risk for more aggressive behavior. Second, it was found that with high teacher sensitivity, less securely attached children are no longer at risk for developing less close relationships with their teachers.