In this thesis, several psychosocial factors associated with internalizing symptoms in adolescence were addressed, with a focus on familial aggregation. Having a parent with mental health problems is one of the most important risk factors for developing psychiatric symptoms, but existing research is limited by the fact that paternal mental health seldom has been examined. In addition, the development and maintenance of internalizing symptoms may rely on individual characteristics of the child, peer factors, socioeconomic status, and other psychosocial factors. Psychosocial correlates can have important implications for both prevention and treatment of internalizing symptoms because such variables can often be modified, in contrast to biological correlates, and may also serve to increase identification of symptoms. Previous research indicates that especially social anxiety is understudied and not easily recognized by adults, even mental health professionals working with youth. The main objectives in this thesis were to examine several psychosocial correlates of internalizing symptoms. In study 1, associations between several mental health symptoms in both parents and internalizing symptoms in adolescent offspring were investigated. In addition, we examined whether parental and offspring sex moderated these associations. In study 2, the associations between recurrent internalizing symptoms in parents over a ten-year time span and internalizing symptoms in adolescent offspring were examined. In addition, we investigated whether associations between parental and offspring internalizing symptoms were mediated by offspring self-esteem and moderated by physical activity. In study 3, we focused solely on internalizing symptoms related to social anxiety and their correlates. All three studies were based on data from the adolescent portion of The Nord-Trøndelag Health Study (Young-HUNT3). The sample in studies 1 and 2 consisted of 5732 adolescents in Young-HUNT3 (ages 13-18) who had one (N=2503) or both parents (N=3229) participating in the adult HUNT3. In study 2, the sample moreover included data from those parents who also participated in the adult HUNT2 when offspring were of a preschool age. This constituted 3198 of the mothers (78%) and 2488 of the fathers (77%). In study 3, the study sample consisted of 7669 adolescents from Young-HUNT3, in addition to comparable data from a clinical sample consisting of 694 participants (ages 13-18) in The Health Survey in the Department of Child and Adolescent Psychiatry, St. Olav’s University Hospital in Sør-Trøndelag, Norway (the CAP Survey). All participants responded to questionnaires reporting on their own mental health. The findings showed that parental symptoms of anxiety and depression, but not alcohol abuse and eating problems, were associated with low subjective well-being, low self-esteem, and more symptoms of depression, general anxiety, and social anxiety in adolescent offspring. None of the associations were dependent on parental or offspring sex, suggesting that internalizing symptoms in fathers and mothers were equally important for offspring symptoms. Parental symptoms of anxiety and depression when offspring were of a preschool age were associated with such symptoms in offspring ten years later, but these associations were fully mediated by current parental symptoms. These findings suggest that the children of parents with internalizing symptoms are at a sustained risk for such symptoms themselves due to the apparent 10-year stability of both maternal and paternal symptoms. The associations between parental and adolescent internalizing symptoms were partly mediated by low adolescent selfesteem, which may be an important precursor of symptoms and amenable to interventions. Adolescent physical activity moderated the association between maternal and offspring internalizing symptoms, and may reduce familial aggregation of such symptoms. This suggests encouraging physical activity in the offspring of parents with such symptoms could be useful. Social anxiety symptoms were frequently reported by adolescents, and the most prominent correlates were academic school problems, bullying, eating problems, and acne problems. These correlates may be easier to detect than social anxiety symptoms, and prevention programs targeting these factors may also be useful for reducing such symptoms. Based on the combined findings, routine screening of a broad range of problems among children, adolescents, and parents may be vital to reduce the burden of internalizing problems in adolescence.