Background: Child externalizing behavior problems such as aggression, hyperactivity, disruptive behavior, and delinquency have been associated with many factors surrounding parenting behavior and parent-child relationships. Parent-child attachment security, in particular, seems to be important for early child development. A child’s secure attachment with their parent has been shown to improve their cognitive, social, and behavioral outcomes. By contrast, insecure parent-child attachment has been found to predict the development of child externalizing behaviors. Harsh parenting may be an important factor in the development of insecure parent-child attachment. Harsh parenting practices (e.g., aggression, corporal punishment, and physical and verbal discipline) can create an environment where children do not receive consistent support during their early child development, which can lead to inconsistent parent-child interactions and externalizing behavior problems. The present study examined parent-child attachment security as a potential mediator and moderator of harsh parenting and child externalizing problems. Method: Participants included a community sample of children (N = 61) ranging from 36-90 months of age. Parents completed a Q-set that measured the security of their attachment with their child. Parents completed self-reported measures of their harsh parenting, and parents and caregivers rated the child’s externalizing behaviors. Results: We hypothesize that child attachment security will mediate the association between harsh parenting and child externalizing behaviors and child attachment security will moderate the relationship between harsh parenting and child externalizing problems. For children with a less secure attachment with their parent, we expect that there will be a strong positive association between harsh parenting and externalizing problems. By contrast, for children with a more secure attachment with their parent, we expect that there will be a weaker association between harsh parenting and externalizing problems. Conclusion: We will draw conclusions after we test the hypotheses and generate findings.