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Depression is a common mental disorder affecting people across the lifespan, with first onset frequently occurring in the teenage years. The disorder is costly to society and constitutes one of the leading causes of disability in youths and adults worldwide. Research demonstrates that depression in childhood or adolescence is linked to adverse adult consequences, including mental health problems, physical health issues, various social difficulties, and economic hardships. While the specific factors and mechanisms associated with these long-term adversities are not well understood, previous studies point to the relevance of considering the heterogeneity in early-life depression. The overarching aim of this doctoral thesis was to shed more light on long-term outcomes of childhood and adolescent depression across multiple life domains. This work made use of extensive follow-up data gathered in Sweden and USA, as part of two community-based longitudinal cohort studies of depressed and nondepressed youths prospectively followed into adulthood. In the Uppsala Longitudinal Adolescent Depression Study, participants were interviewed around age 16 (n=631) and age 31 (n=409). Using linkage to nationwide population-based registries, participants were followed up around age 40 (n=576). In the Great Smoky Mountains Study, participants were interviewed at repeated occasions in childhood and adolescence (n=1,420), and at further follow-ups in adulthood extending up to age 30 (n=1,336). Findings from this work suggest that childhood/adolescent depression can have long-lasting associations with a broad spectrum of adverse outcomes. First, the risk of adult depression is known to be elevated among those exposed to depression in early life; however, depressed youths experiencing major conflicts with parents may be at an additionally increased risk of subsequent depression recurrence. Second, early-life depression was found to be associated with higher levels of adult psychiatric disorders, and also with worse health, criminal, and social functioning, even when accounting for a multitude of potential confounders. Third, early-life depression was predictive of poor labor market outcomes, especially for those with persistent depression. This link was partially mediated by the course of depression. Fourth, the welfare burden associated with early depression amounted to considerable public expenditures in adulthood, particularly for those with persistent depression or comorbid psychiatric conditions such as anxiety and disruptive behavior disorders. The adverse long-term consequences in the wake of early-life depression underscore the importance of prevention and treatment approaches that are both efficacious and cost-effective. Such targeted efforts may have the potential to avert later ill-health, impairment, and possibly also economic disadvantage.