Muscle dysmorphia (MD) was discovered by Pope, Katz, and Hudson in 1993 in a study aimed at analyzing the use of anabolic steroids in a sample of 108 bodybuilders in the United States. At the beginning of the study of this "new" mental disorder, different theoretical models were proposed in order to approach its etiology.Mental disorders appear in medical manuals and books, each of them classified according to different diagnostic criteria. The classification of the disorder is accompanied by the description of its most frequent symptomatology and the proposal of a potentially effective treatment. Until 2002, owing to the unclear etiology of MD and the lack of consensus about its classification, medical manuals did not categorize it as a specific disorder. Although MD presents a high comorbidity with eating disorders (EDs), obsessive-compulsive disorder (OCD), and body dysmorphic disorder (BDD), it has been eventually classified as a part of the latter in the following systems: "Diagnostic and Statistical Manual of Mental Health Disorders, 5th edition" (DSM-5, American Psychiatric Association, 2013), the "International Classification of Diseases for Mortality and Morbidity Statistics" (ICD; World Health Organization, 2018), and the "Merck Manual," 19th edition (Porter & Kaplan, 2011).The approach to the etiology of this disorder is still a handicap for its research. So far, the instruments designed and validated in different backgrounds and cultures appearing in the meta-analysis by Rubio-Aparicio, Badenes-Ribera, Sánchez-Meca, Fabris, and Longobardi (2019) have been of utmost importance in order to discover the elements and factors that intervene in the development of this condition. Consequently, are the available instruments used to assess MD reliable despite the lack of an in-depth knowledge of its etiology?