An eating disorder is a mental illness in which a person eats in a way that disturbs their physical health. Overeating is the most common and obvious disorder, and was in the past often attributed to a lack of self-control. Psychologists prefer to class the other syndromes as mental illness, going by the mental health model that views the syndrome as caused by something largely outside human will. Seen this way, these disorders are said to interfere with normal food consumption and lead to serious health problems. Patients diagnosed with Bulimia nervosa and Anorexia nervosa have a mortality rate of between 5% and 12% per decade, which is a higher mortality rate than any other mental illness.

People whose eating is disordered in these ways experience psychological suffering, typically becoming obsessed with food, dieting and often body image, and their health is at extreme risk due to malnutrition. In the prevailing psychological view, patents with an eating disorder are seen as victims rather than as conscious actors; their suffering is not seen as self-inflicted but as the result of a disease. Most people with an eating disorder attempt to hide their abnormal behavior from others. They do not accept the diagnosis and will refuse treatment. As the treatments prescribed for eating disorders can take decades, mental health advocates warn that early identification of these disorders (and diagnosis of the syndrome as being caused by mental illness) may be the difference between life and death for the patient.

The two most familiar types of eating disorder are anorexia nervosa and bulimia nervosa. Binge eating disorder is similar to bulimia. Large amounts of food are consumed at a sitting but retained in the stomach rather than being regurgitated. Some psychologists also classify a syndrome called Orthorexia as an eating disorder - the person is overly obsessed with the consumption of what they see as the 'right' foods for them (vegan, raw foods, etc), to the point that their nutrition and quality of life suffers. Some people have food phobias about what they can and can't eat, which some also call an eating disorder. Another condition, which is somewhat qualitatively different from the foregoing, is Pica, or the habitual ingestion of inedibles, such as dirt, wood, hair, etc.

There are many different degrees of anorexia and bulimia. Anorexia may be mild, where a person may eat but only allow themselves certain foods, or severe, to the point of literally starving themselves to the point of death. There are other forms of purging besides vomiting- compulsive exercise, laxatives, and enemas being the other main ones. Other sufferers also allow themselves a small variety of foods and exercise compulsively, being categorized as eating disorder not otherwise specified (ED-NOS).

Women account for 90% of eating disorder cases. Anyone can have an Eating Disorder, but it most often occurs with young teens because they are the most at risk, due to their extreme exposure to the media. Teen’s feelings of need to conform to an idea of beauty that the media projects can be too much for some to deal with and can prompt them to take drastic measures to change their appearance.