Eating Disorders (EDs) constitute a serious public health issue that affects mainly women and appears typically in adolescence or early adulthood. EDs often result in serious physical health consequences as well as enduring psychological difficulties and behavioral complications. Prevention of EDs is paramount as it will reduce such adverse effects, improve the quality of the patient's and his/her family's life and decrease costs incurred by EDs. To date, prevention approaches have not been uniformly effective for EDs and as a consequence there is great room for improvement. There is a growing body of evidence supporting the functional relationship between EDs symptomatology and controlling emotional states either by avoiding or inhibiting the emotional response. Furthermore, theories support that AN begins with a general need of self control and not just emotional control. Current prevention programs for EDs have largely ignored on the problem concerning self-control. This study will propose a new prevention program based on the cognitive-behavioral theory, which suggests that one of the main features of anorexia is a general need of self-control, and the psychological inflexibility theory, which suggests that this need of experiential control may lead to a broader state of psychological inflexibility. A prevention program based on mindfulness, acceptance and change is hypothesized to be able to target ineffective control strategies in individuals at high-risk for developing EDs, prior to the disorder manifesting itself. A treatment which is based on mindfulness and acceptance is Acceptance and Commitment Therapy (ACT), a 3rd wave CBT that targets ineffective control strategies and inflexibility found in a range of psychopathology problems. This study will apply an ACT based prevention program for EDs in an internet based delivery modality delivered to high-risk individuals. The hypotheses made are the following: participants receiving the prevention program will 1) develop greater body image flexibility, 2) decrease their control behaviours (e.g. dieting, exercising, body image avoidance behaviors), and 3) have significantly fewer ED symptoms, decreased body dissatisfaction, decreased risk for developing EDs and have significantly higher quality of life, as compared to the wait-list control group at the end of prevention and at 1-month follow up. We also hypothesized that changes in ACT-related process measures (defusion, mindfulness, values, willingness, body image flexibility, self as context) will mediate improvements in EDs outcome measures (eating disorder risk, eating disorder symptoms, dieting, exercising, body image avoidance behaviour). For the purposes of the study, high-risk participants developing EDs will be recruited from high schools and vocational schools. Participants will be randomly assigned either to the internet-based ACT intervention or the wait list control group. Participants will complete process and outcome measures at baseline, post treatment and follow-up. Results will be then analyzed and conclusions will be made. Repeated measures MANOVA will be used to assess the changes and the differences between the prevention group and the wait list control group. Furthermore, to assess whether ACT variables such as values, mindfulness, defusion, self as context, willingness will mediate ED symptoms mediation analysis will be used.