The feasibility and efficacy of Internet treatment programs for overweight and obese people have been demonstrated in a series of randomized trials. Initial studies examined various approaches to Internet behavioral treatment. Other studies have examined delivery of group behavioral counseling using Internet chat rooms, using the Internet for long-term maintenance of weight loss, and enhancing motivation in Internet programs. These interventions have produced weight losses of 4-7 kg over 6 months to 1 year when support via e-mail, automated messages, or chat rooms is provided. Outcomes and lessons learned with application to the treatment of substance use and misuse are provided.