This paper examines the relative health and behavioural effects of maternal, paternal, and dual-parent substance abuse on a cohort of three-year-old children. It finds that substance abuse by either parent has a significant negative effect on children's development, and that this negative effect is greater if both parents have substance abuse problems. In addition to this, the study suggests that substance abuse by a resident parent is more detrimental than substance abuse by a non-resident parent.