The current study examined the mutual influence of parenting and boys' externalizing behavior from 4th to 8th grade, how these relationships change as children develop, and the stability of parenting and child behavior in a sample of 122 boys. Child behavior predicted poor parental monitoring at 6th and 7th grade and inconsistent discipline at all grade levels examined. Parenting behavior was not related to child behavior above and beyond the stability of child behavior. Stability of child behavior decreased from 5th to 6th grade and stability of parental monitoring decreased from 5th--6th and 6th--7th grade, suggesting that 6th grade was an important transition point for both parenting and child behavior. Implications of these findings for prevention and intervention are discussed.