Half of all mental health disorders begin by the age of 14, with significant effects both for the individual and for wider society. As such, adolescence is a critical period for the prevention of and intervention in mental health difficulties. In hopes of developing new treatments and improving pre-existing therapies, there has been much investigation of potential “active ingredients” by which therapeutic change may be driven. One of these active ingredients may be psychological decentering, a self-reflective capacity featuring across a spectrum of psychological therapies. Psychological decentering is a skill through which distressing internal events, including difficult thoughts, feelings, and memories, are re-perceived from an observer perspective. This typically results in a reduction in identification with these experiences, and attenuates emotional reactivity. This skill has been hypothesised to be protective against mental health difficulties. The work presented in this thesis attempted to answer four core research questions: (1) what are the characteristics of decentering in an adolescent sample? (Chapters 2, 3 & 4) (2) How is decentering connected to adolescent mental health? (Chapters 2, 3 & 4) (3) What cognitive processes are associated with decentering? (Chapters 3 & 4) And (4) can decentering be trained and does this impact mental health in adolescence? (Chapters 5 & 6). This was achieved using a narrative review, regression models calculated using data from two cross-sectional samples (N = 553 and N = 904), and a preliminary randomised controlled trial comparing low-intensity remote decentering skills training against an active control condition (N = 115). There was conflicting evidence for the association between age and decentering across adolescence, with one sample suggesting a positive association, and one a negative. There was consistent strong evidence for relationships between decentering and mental health symptoms, in which higher self-reported decentering was associated with fewer symptoms, and better wellbeing. There was also evidence of association between decentering and emotion regulation strategies, with higher self-reported decentering predicting broader use of emotion regulation strategies, and fewer difficulties with emotion regulation. In terms of cognitive processes and their association with decentering, there was consistent evidence of a lack of association between decentering and cognitive control on a range of tasks. However, there was evidence of association between decentering and a self-referential processing task, in which participants with higher self-reported decentering committed fewer errors on a condition in which they were required to manipulate self-perspective. Finally, a randomised controlled trial conducted with adolescents with elevated depression symptoms showed good preliminary evidence of improving individuals self-reported decentering, reducing depression symptoms, and increasing psychological wellbeing. In conclusion, the work presented in this thesis provides further evidence of decentering as a predictor of broader mental health and emotion regulation in adolescence. It is currently unclear how this skill may develop across this period, with further longitudinal studies needed. Most importantly, a fully-powered randomized-controlled trial shows promising evidence that, compared with an active control, decentering can be trained, using a remote, low-intensity intervention. This improvement in decentering coincides with a significant reduction in depression symptoms. However, a larger effectiveness trial is needed to confirm the benefits of this intervention, and to investigate possible moderators and mediators of the observed effects. This thesis offers promising possibilities for advancing interventions for young people, and uncovering how these treatments function.