Self-help Internet interventions have the potential to enable consumers to play a central role in managing their own health. This paper contains a systematic review of 15 randomised controlled trials of the effectiveness of self-help Internet interventions for mental disorders and related conditions. Conditions addressed by the interventions included: depression, anxiety, stress, insomnia, headache, eating disorder and encopresis. Most interventions were reported to be effective in reducing risk factors or improving symptoms, although many of the studies had methodological limitations. Three of the interventions that reported positive outcomes are available without charge to the public. (Contains 2 tables.)