The upward trend in the prevalence of childhood mental disorders observed in the UK over the previous two decades, together with UK's poor performance in recent international comparisons of child well-being, has brought childhood social and emotional well-being (SEW) to the forefront of policy. Key to tackling this issue is to understand what causes SEW in childhood, what interventions are successful in improving it, and what are its late-life consequences. This thesis furthers the literature in each of these areas.Firstly, we examine whether foetal (or in-utero) exposure to influenza hampers the development of childhood SEW. To do so, we examine the use of an instrumental variables approach, whereby the severity of the 1957 Asian Flu epidemic in the local authority of birth is used as an instrument for whether mothers self-report contracting influenza during pregnancy. We establish that exposure has little effect on childhood SEW, but that it results in a 60% increase in the risk of being stillborn, suggesting an increasing focus on influenza vaccination during pregnancy is needed.Secondly, we investigate the long-term effectiveness of school-based interventions to improve SEW. In order to overcome the absence of long follow-up in trial datasets, we develop a new modelling approach which involves the matching of trial participants to individuals in birth cohort datasets. An application of this method found that a Promoting Alternative Thinking Strategies (PATHS) intervention implemented in Greater Manchester schools led to a statistically significant improvement in childhood SEW, and had a positive, although statistically insignificant, effect on health across the life-course.Finally, we address the paucity of studies examining the effects of childhood SEW on late-life health and labour market outcomes. To do so, we develop a method for generating predictions of the effects of childhood characteristics beyond the currently available follow-up periods in birth cohort datasets, adapting an existing mediation analysis framework. Applying this method, we establish that a one-standard deviation improvement in childhood SEW leads to an increase of up to 0.18 accumulated quality-adjusted-life-years in late-life, and an increase in pre-tax labour income in late-life of up to £23,850. Both of these effects are primarily driven by large positive effects of childhood SEW on educational attainment, employment, income and health in mid-life.Childhood SEW is a predictor of important outcomes throughout the lifecourse. More research is needed to identify its causes and interventions to successfully improve it.