The aim of this thesis was to explore social capital and inequalities in mental health among young adolescents in Sweden. This is a compilation thesis comprising four studies. Studies I and II are quantitative studies of crosssectional data from the Swedish Health Behaviour in School-aged Children survey. The aim of Study I was to investigate socioeconomic inequalities in health using both a subjective and an objective measure of socioeconomic status among Swedish adolescents. The findings showed that subjective socioeconomic status robustly and independently predicted mental health problems, poor life satisfaction and poor general health perception. The association between objective socioeconomic status and mental health was weakened, and even reversed, when subjective socioeconomic status was accounted for in regression models. A Latent Profile Analysis was applied in Study II with the aim of identifying distinct profiles of family, school and peer social capital in a nationally representative sample of adolescents and to explore health outcomes in those profiles. The findings showed that five distinct profiles best represented the data for 11 and 15-year olds, while a four-profile model was optimal for 13-year olds. Significant inequalities were identified between profiles when these were examined in terms of mental health problems and life satisfaction. The design of Study III was a qualitative semi-structured interview study. The aim was to explore social capital from the perspective of adolescents in relation to mental health. Adolescents spoke of having access to a safe space, feeling connected to others and predictability as important aspects of social relationships and networks in relation to mental health. The aim of Study IV was to identify and evaluate the design and psychometric properties of instruments for assessing social capital specifically developed and validated for self-reporting among adolescents (10-19 years). The design was a systematic review, in which 20 instruments were identified. The results revealed a lack of instruments that covered both the multidimensionality of social capital and contextual relevance in relation to adolescents. The conclusion from this thesis is that social capital may be useful for identifying vulnerable individuals and for differentiating between the natural imbalance of adolescence and what may lead to serious illness. Longitudinal research and refinement of the operationalization of the concept are, however, needed to enhance the understanding of these findings.