Internet therapy is a novel treatment approach that is used to deliver cognitive behaviour therapy. Treatment components are mainly delivered in the form of texts presented via webpages, and support is provided via e-mail. A growing number of controlled trials suggest that Internet therapy works well when (a) a proper diagnosis is made before the treatment starts, (b) a comprehensive treatment is provided, (c) the treatment is user friendly and not overly technically advanced, and (d) support and a clear deadline are provided for the duration of the treatment. Several issues remain for exploration in future research, such as mediating and moderating mechanisms and the role of tailoring the intervention.