**1. Introduction**

Weight management is essential for a healthy life, but in extreme cases, it can turn into a life-threatening condition. Eating behavior is an important dimension of weight management. For most of us, eating is an automatic response to hunger and can be as easy or normal as breathing. On the other hand, it may be a challenging area for people with eating disorders. Eating disorders (EDs) are serious psychiatric problems that have a multiple impact on health and well-being. The prominent types of EDs include anorexia nervosa (AN), bulimia nervosa (BN), and binge eating disorder (BED). This chapter will focus on anorexia nervosa and start with a brief history of AN. In the following sections, basic diagnostic criteria and a review of comorbid psychiatric and medical conditions will be addressed. Throughout the text, we will discuss prominent theories regarding its etiology and treatment options from a biopsychosocial perspective. Finally, prevention studies will be highlighted.

Anorexia nervosa is a complex disorder that includes physiological, behavioral, cognitive, and emotional components. Historical traces of anorexia can be found in ancient times. A group of women who starved themselves for religious reasons in Rome in 383 was reported [1]. Fasting is a common ritual in many religions and cultures, although starving triggered by psychological factors as a weight management strategy can lead to serious medical problems. Cases similar to AN have been reported since the fourteenth century, but as a psychological problem, preliminary cases were defined in 1873 and 1874 [2, 3]. The term anorexia nervosa means "nervous loss of appetite"; thus the early descriptions focused on food avoidance as the core problem. Then it was realized that people with AN do not suffer a loss of appetite; indeed their mind is extremely preoccupied with food. Hence, the psychological component became prominent, and the problem was conceptualized as a weight phobia and self-control. In fact, AN has been known about since the seventeenth century but was observed in the 1960s in western society and characterized as leading to a significantly low body weight because of restricting energy intake or compensating to an excessive rate intentionally, in order to attain or maintain an unrealistically thin ideal weight [4]. In the fifth edition of the *Diagnostic and Statistical Manual of Mental Disorders* (*DSM-5*), the title Feeding and Eating Disorders covers problems related to eating behaviors and unhealthy strategies for weight management [5]. Feeding disorders include pica, rumination disorder, and avoidant/restrictive food intake and can usually be seen in children, resulting in malnutrition or delay in growth due to unhealthy feeding behaviors. On the other hand, EDs are mostly seen in teenagers and adults. Their onset usually falls during puberty when body changes gain importance. DSM-5 defines three types of EDs, AN, BN, and BED. The underlying psychological mechanism is similar between these types as an intense fear of gaining weight and preoccupation with weight, body, and eating that leads to weight management strategies also known as compensatory behaviors like dieting, exercise, self-induced vomiting, misuse of laxatives, and diuretics [6]. The subtypes of EDs differ in body weight and weight management strategies. BN, BED, and other problems related to eating are beyond the scope of this chapter. Thus, we will first take a closer look at the clinical presentation of anorexia nervosa.
