This research describes the design and evaluation of an internet-based universal program for use in schools with adolescent students to prevent common mental disorders and promote mental health. The research began in response to investigations that showed that rates of mental illness in Australian children, teenagers and adults were high, that these illnesses caused significant burden to individuals and society, and that there were insufficient services to treat. When current interventions are unable to alleviate disease burden it is important to focus on prevention. Mental health prevention should target youth before disorders cause disability and restriction of life choices. A review of the mental health prevention literature supported a universal cognitive behavioural approach in schools. Internet delivery was used to maintain content integrity, enable access to people living in regional and remote areas, and to appeal to young people. Internet delivery makes universal prevention cost effective and feasible. The Intervention Mapping approach was used to direct the design of the program. A feasibility study was conducted to gain opinions from students and teaching staff. Changes were made in light of results from this study and 463 students were then exposed to the program in an effectiveness trial. The effectiveness trial was a before-after design with no control group. Results from this trial provided evidence that the program was acceptable and effective for use by teachers in the intervention schools. Also student behaviour and mood changed in beneficial ways after program administration. Specifically, student reported significantly increased knowledge about stress and coping, use of help-seeking behaviours, and life satisfaction, and significantly decreased use of avoidance behaviours, total difficulties and psychological distress. The study design allows causal inferences to be surmised concerning exposure to the intervention and changes in behaviour and mood, but further evidence is needed before firm conclusions about effectiveness can be posited and generalizations made concerning different populations, settings and times. In conclusion, this thesis provides evidence that a computerised, cognitive behavioural mental health prevention program delivered to adolescent school students by teachers can potentially change student coping behaviours and mood in beneficial ways.