PROJECT SUMMARY/ABSTRACT At least 1 in every 10 children is exposed to maternal depression. Depression during adulthood is one of the leading causes of disability, and when it occurs among women with children, depression also carries a 3- to 6-fold increase in risk for offspring internalizing and externalizing behavior problems. Children's behavior problems, in turn, elevate risk for subsequent maternal depression. The intertwined, transactional relations between maternal depression and children's behavior problems call for integrated maternal-child mental health interventions. However, integrated programs are scarce; adult and child mental health treatment programs typically operate in narrow silos. For example, Attachment and Biobehavioral Catch-up (ABC) is an empirically-supported 10-session home-visiting parenting intervention that focuses on improving maternal sensitivity. ABC reduces children's behavior problems, but it does not treat maternal depression. Similarly, Mothers and Babies is an empirically- supported cognitive behavioral intervention for depression that focuses on improving mothers' negative thinking and behavioral activation. Mothers and Babies reduces perinatal maternal depression, but it does not treat children's behavior problems. Programs that address both maternal and child well-being are essential. The proposed project is a randomized controlled trial to evaluate the efficacy of an integrated intervention that dually treats maternal depressive symptoms and offspring behavior problems. Families (n = 240 mothers and their 2-to-4 year-old children) will be randomized to receive 10 sessions of ABC or 10 sessions of ABC+D in their home. ABC+D is an expanded version of ABC that additionally treats maternal depressive symptoms using Mothers and Babies adapted for mothers of toddlers. The specific aims of this proposed project are: Aim 1 – Evaluate the efficacy of ABC and ABC+D for reducing maternal depressive symptoms and child behavior problems. We will also determine the extent to which reductions in maternal depressive symptoms are a function of reductions in child behavior problems and vice versa, and Aim 2 – Test whether the effects of ABC+D on maternal depression and child behavior problems are mediated through decreases in maternal negative thinking, increases in maternal behavioral activation, and improved maternal sensitivity. Primary outcomes are maternal depressive symptoms and children's behavior problems. Our long-term goal is to contribute to precise interventions that are tailored to mental health problems at the level of the family. The proposed project has high public health significance for reducing the burden of maternal depression and children's behavior problems and for disrupting the transactional relations that perpetuate them over time.