Optimal mental health is a global and increasingly pressing issue. Major events in recent times, such as the effects of global warming and the Covid-19 pandemic, have had undeniable negative effects on people’s well-being. The mental health of young people is of particular importance: strategies to deal with adversity are learned and developed at this stage of life. Success aids the likelihood of being a lifelong resilient and hopefully flourishing individual, whereas difficulty may leave an individual vulnerable to slipping into persistent mental ill health. Given this, there seems an obvious and necessary role for educational institutions in promoting such strategies and thus improving the well-being of students. Positive psychology has a valuable contribution to make in this regard, as achieving optimal mental health is perhaps its primary purpose, and positive psychology interventions (PPIs) are broad-based and cost-effective to implement when compared with more traditional clinical approaches. They are also technology friendly, meaning they may be disbursed widely. The research in this thesis explores the use of several PPIs in school and university settings with the aim of building significant and lasting well-being improvements for young people. It works to find the most effective interventions and to understand what makes them so. Finally, it seeks to create real-world value by considering how such interventions may be designed and implemented in an educational context. The thesis comprises four studies. The first study used a positive diary exercise in a sample of primary school children. The exercise resulted in a significant increase in happiness and reduction in depressive symptoms during and up to three months after the intervention. A tertile split revealed interestingly different response profiles for participants depending on baseline well-being. A similar intervention in the second study found significant associated improvements in academic performance. The third study applied two PPIs in samples of undergraduates. The first part also used the diary exercise, which found there to be less convincing evidence of its effectiveness when compared with the first two studies. The second part used a signature strengths intervention, which resulted in marked and sustained improvements in self-esteem and life satisfaction. This study highlighted the differing outcomes that different interventions may have, particularly across age groups. It also raised questions as to why certain well-being markers improve more or last longer than others, suggesting that an individual’s sense of autonomy or control may be a factor. These questions shaped the fourth study, which was more exploratory. It used a novel exercise-framed PPI to determine whether a sense of control, as measured by judgments of contingency, is linked with well-being, and whether positively manipulating such judgments might lead to improvements thereof. The complexity of the experimental paradigm meant that findings were inconclusive in this regard, but it opened the door to future work that might be able to distil more robust effects. Overall, this thesis finds that PPIs demonstrate huge promise for mental health improvements and that they are a feasible option for incorporation into an educational curriculum. However, it seems they are not universal aids and careful consideration needs to be given to the type of intervention used and who the target recipients are. Age and baseline well-being are two important factors, for example. If properly implemented, PPIs could be valuable tools to build resilience and enable young people to flourish now and through the rest of their lives.