Informed by the transactional and bioecological theories, this study examined the bidirectional relations between intrusive caregiving and children's externalizing behaviors across the family and school systems during the early elementary school years. Using data from the NICHD Study of Early Child Care (N=1364), these bidirectional associations were examined from 54 months of age through third grade. Both mothers' and teachers' intrusive caregiving behaviors contributed to the development of children's externalizing behaviors in the first three years of elementary school, and children's externalizing behaviors consistently predicted mothers' intrusive caregiving behaviors. Children's externalizing behaviors mediated the relation between teachers' intrusiveness and mothers' intrusive caregiving. These bidirectional associations did not vary across child gender, but tended to be stronger among non-White children than their White peers. These findings suggest that children's own behaviors can serve as a key connection between the two core institutions of child development--the family and school systems. (author abstract)