This is a doctoral thesis comprising a systematic literature review/meta analysis and an empirical paper. SYSTEMATIC LITERATURE REVIEW The efficacy of self-guided internet and mobile-based interventions (IMIs) for preventing anxiety and depression – A systematic review and meta-analysis Abstract Background: Anxiety and depression are highly prevalent mental disorders which are associated with a considerable personal and economic burden. As treatment alone has a minimal impact on prevalence, there is now a growing focus on interventions which may help prevent anxiety and depression. Internet and Mobile based Interventions (IMIs) have been identified as a useful avenue for the delivery of preventative programmes due to their scalability and accessibility. The efficacy of IMIs that do not require additional support from a trained professional (self-guided) in this capacity is yet to be explored. Method: A systematic search was conducted on the Cochrane Library, PubMed, PsycARTICLES, PsycINFO, OVID, MEDline, PsycEXTRA and SCOPUS databases. Studies were selected according to defined inclusion and exclusion criteria. The primary outcome was evaluating the effect of self-guided IMIs on incidence of anxiety and depression. The secondary outcome was effect on symptom severity. Results: After identifying and removing duplicates, 3211 studies were screened, 32 of which were eligible for inclusion in the final analysis. Nine studies also reported incidence data (depression = 7, anxiety = 2). The overall Risk Ratios (RR) for incidence of anxiety and depression was .86 (95% CI [.28, 2.66], p =.79) and .67 (95% CI [.48, .93], p =.02) respectively. Analysis for 27 studies reporting severity of depressive symptoms revealed a significant posttreatment standardised mean difference (SMD) of −.27 (95% CI [ −.37, −.17], p < .001) for self-guided IMI groups relative to controls. A similar result was observed for 29 studies reporting severity of anxiety symptoms with an SMD of -.21 (95% CI [-.31, -.10], p < .001). Conclusions: Self-guided IMIs appear to be effective at preventing incidence of depression, though further examination of the data suggests that generalisability of this finding may be limited. While self-guided IMIs also appear effective in reducing symptoms of anxiety and depression, their ability to prevent incidence of anxiety is less clear. Keywords: Internet, mobile-based, prevention, depression, anxiety, symptoms, incidence EMPIRICAL STUDY Evaluating the effects of a self-help mobile phone application on worry and rumination experienced by young adults: A randomised controlled trial. Abstract Background: Improving the mental health of adolescents and young adults has been identified as a global priority. As well as preventing poor mental health, this includes the promotion of increased well-being by developing interventions which target known risk factors such as worry and rumination, also known as Repetitive Negative Thinking (RNT). Objective: The primary objective of this study was to test whether an RNT-targeting self-help mobile phone application (MyMoodCoach) could reduce levels of worry and rumination in young adults currently residing in the United Kingdom. A secondary objective was to test whether the app could achieve similar reductions in symptoms of anxiety and depression, as well as improve well-being. Methods: A web-based, single blind, two-arm parallel-group Randomised Controlled Trial (RCT) was conducted with N=236 people aged between 16 and 24 who self-reported high levels of either worry or rumination. Participants were recruited via social media adverts and directed to an open website. Eligible participants were randomised to an active intervention group (usual practice, plus up to 6 weeks of using the RNT-targeting mobile app) or a waitlist control group (usual practice with no access to the app). The primary outcome was changes in worry and rumination six weeks after randomisation. Secondary outcomes included changes in well-being and symptoms of anxiety and depression after six weeks and changes on all measures after 12 weeks. The waitlist control group was given access to the app at the six-week follow-up point. Results: Participants were randomised to access the RNT-targeting app (n = 119) or waitlist control (n = 117). The active treatment arm showed significantly lower levels of rumination (-2.92, 95% CI [-5.57, -.28], p <.031, ηp2 = .02) and worry (-3.97; 95% CI [-6.21, -1.73], p <.001, ηp2 = .06) at six-week follow-up, relative to the waitlist control. Similar differences were observed between the two arms for wellbeing (p <.001) and symptoms of anxiety (p = .03) and depression (p = .043). The waitlist control reported similar levels of improvement when given access to the app after six weeks. Improvements observed in the active treatment arm were maintained at the 12-week follow-up point where there were no significant differences between the two arms. In terms of adherence, 97 out of 119 participants in the active treatment arm (81.5%) and 53 out of 96 (55.2%) of the waitlist control signed into the app. Conclusions: The MyMoodCoach app had a significant positive effect on worry and rumination, well-being, anxiety, and depression in young adults, relative to waitlist controls. This suggests it has the potential to target a known prevention mechanism. Further research is needed to examine effect on incidence and whether the app can provide longer term benefits.