Ours is a society in which thinness, particularly in women, is idealised, even at the cost of health. Adolescent girls and young women are especially at risk of developing eating disorders such as anorexia nervosa, bulimia nervosa, and binge eating disorder. The need for wide-spread prevention among at-risk populations is paramount, as these disorders are often difficult to treat and can contribute to a range of physical and mental health problems. Studies have found that a cognitive dissonance-based intervention significantly outperforms other intervention programs and is successful in preventing onset of eating disorders. This facilitator guide outlines a two part group intervention program for adolescent and college-aged girls at risk of developing eating disorders. In the first part of the program, participants critique the thin-ideal through discussion, role-playing, and written exercises. Participants learn skills that increase body satisfaction, decrease unhealthy weight control behaviours, and prevent eating disorder symptoms. The second part of the intervention is designed to help participants make gradual and permanent lifestyle changes to achieve a healthy body weight. It teaches how to eat for energy balance, make healthy food choices, and incorporate physical exercise into a daily routine. This group therapy program is based on 16 years of research and has been completed by over 1,000 adolescent girls and young women. It can be effectively delivered by 'real world' providers, such as school counsellors, nurses, and teachers. This facilitator guide provides all the information needed to successfully implement the program, including explanation of Cognitive Dissonance theory, session outlines complete with exercises, and recommendations on how to train group leaders and recruit participants.