PROJECT SUMMARY/ABSTRACT Approximately 1 in 7 women experience perinatal depression (i.e., during pregnancy or the postpartum period), making it the most common complication of childbirth. Perinatal depression is associated with long-lasting consequences, including increased risk of suicide, impairments in parenting, and immense societal costs. Prenatal insomnia is a robust risk factor for perinatal depression: nearly 1 in 4 women who experience prenatal insomnia develop postpartum depression. To date, depression prevention interventions mainly focus on pregnant women with elevated depressive symptoms or a history of depression, and no trials have investigated whether treating prenatal insomnia prevents perinatal depression. Digital cognitive behavior therapy (CBT-I) is safe and effective for treating prenatal insomnia and shows promise for preventing perinatal depression relative to standard care. Further, digital CBT-I may be of particular interest for pregnant women because it minimizes wait time, avoids burdensome traveling and scheduling requirements, and meets their preferences for flexible delivery options. The proposed project is a blinded randomized controlled trial to evaluate the efficacy of digital CBT-I for the prevention of depression during pregnancy and through 12 months postpartum among 498 non-depressed women with insomnia disorder relative to a credible, clinically-relevant control condition. Consistent with other research, sleep hygiene education will be the control condition, and will match digital CBT-I in delivery format (digital), frequency (weekly), and number of sessions (six). The proposed confirmatory efficacy trial addresses three specific aims: 1) To evaluate the efficacy of digital CBT-I for preventing perinatal depression; 2) To test whether the effect of digital CBT-I on perinatal depression is mediated through prenatal insomnia symptom improvement; 3) To test whether the effect of digital CBT-I on perinatal depression is moderated by baseline depressive symptom severity. By focusing on a low stigma target (i.e., prenatal insomnia) and using a scalable intervention, this approach has the potential to facilitate broad dissemination. The proposed project has high public health significance for reducing the burden of perinatal depression for mothers, families, and society.