Context: The familial nature of childhood conduct problems has been well documented, but few genetically informed studies have explicitly explored the processes through which parental conduct problems influence an offspring's behavior problems. Objective: To delineate the genetic and environmental processes underlying the intergenerational transmission of childhood conduct problems. Design: We used hierarchical linear models to analyze data from a Children of Twins Study, a quasiexperimental design, to explore the extent to which genetic factors common to both generations, unmeasured environmental factors that are shared by twins, or measured characteristics of both parents confound the intergenerational association. Setting: Participants were recruited from the community and completed a semistructured diagnostic telephone interview. Participants: The research used a high-risk sample of twins, their spouses, and their young adult offspring (n = 2554) from 889 twin families in the Australian Twin Registry, but the analyses used sample weights to produce parameter estimates for the community-based volunteer sample of twins. Main Outcome Measure: Number of conduct disorder symptoms. Results: The magnitude of the intergenerational transmission was significant for all offspring, though it was stronger for males (effect size [Cohen d] = 0.21; 95% confidence interval, 0.15-0.17) than females (d = 0.09; 95% confidence interval, 0.05-0.14). The use of the Children of Twins design and measured covariates indicated that the intergenerational transmission of conduct problems for male offspring was largely mediated by environmental variables specifically related to parental conduct disorder (d = 0.13; 95% confidence interval, 0.02-0.23). In contrast, the intergenerational transmission of conduct problems was not because of environmentally mediated causal processes for female offspring (d = ?0.09; 95% confidence interval, ?0.20 to 0.03); a common genetic liability accounted for the intergenerational relations. Conclusions: The mechanisms underlying the intergenerational transmission of conduct problems depend on the sex of the offspring. The results are consistent with an environmentally mediated causal role of parental conduct problems on behavior problems in males. Common genetic risk, however, confounds the entire intergenerational transmission in female offspring.