Background: The differential effectiveness of parent training has led researchers to examine a variety of child, parent, and familial variables that may predict treatment response. Studies have identified a diverse set of child, parent psychological/behavioral and demographic variables that are associated with treatment outcome and dropout. Method: The parent training literature was examined to isolate child, parent, and family variables that predict response to parent training for child externalizing behavior problems. A literature review was conducted spanning articles published from 1980 to 2004 of indicated prevention (children with symptoms) and treatment (children with diagnosis) studies. Meta-analyses were conducted to determine standardized effect sizes associated with the identified predictors. Results: Many of the predictors of treatment response examined in this meta-analysis resulted in moderate standardized effect sizes when study results were subjected to meta-analytic procedures (i.e., low education/occupation, more severe child behavior problems pretreatment, maternal psychopathology). Only low family income resulted in a large standardized effect size. Predictors of drop-out resulted in standardized effect sizes in the small or insubstantial range. Conclusions: Response to parent training is often influenced by variables not directly involving the child, with socioeconomic status and maternal mental health being particularly salient factors.