This project aimed to develop a digital web and mobile phone intervention for reducing Estonian adolescent and young adult alcohol and tobacco consumption. A systematic review was conducted, including a meta-analysis based on 32 randomised controlled trials, to investigate the associations with effectiveness of digital interventions in reducing adolescent and young adult alcohol and tobacco consumption. Digital interventions reduced adolescent and young adult weekly drinking (mean difference = -0.55, 95% CI (-1.04, -0.05), I2=93%) and monthly binge drinking (mean difference = -0.30, 95% CI (-0.55, -0.05), I2 = 75%). Digital interventions increased smoking cessation (risk ratio = 1.70, 95% CI (1.37, 2.11), I2= 35%). A qualitative focus group study with Estonian adolescents and young adults (N=22) indicated a lack of knowledge regarding effects of alcohol and tobacco consumption early in life and a recognition of the difficulty to change alcohol and tobacco consumption. A web and mobile phone based intervention programme was appealing to the focus group participants. The development of the first individually tailored web and mobile phone intervention targeting Estonian adolescent and young adult alcohol and tobacco consumption was undertaken. The content of this intervention was informed by the systematic review and meta-analysis, focus group study, psychological theory, and participatory design. The intervention, called MyOwnMe, is a tailored web program linked to a daily mobile phone text-messaging program. A pilot study with Estonian adolescents and young adults (N=22) indicated feasibility of implementation in Estonia and acceptability of intervention content. No difference was found between the intervention and control group in alcohol (mean difference = -0.2 95% CI (-0.9, 0.6), p = 0.62) or tobacco consumption (30-day abstinence from cigarette smoking RR = 1.25, 95% CI (0.81, 1.94)) after the 8-week study period. Results of this pilot study will be used for recommendations in this thesis on the development of individually tailored web and mobile phone interventions for Estonian adolescents and young adults.