Eating disorders have been defined as “characterized by persistence disturbance of eating or eating-related behavior that results in the altered consumption or absorption of food and that significantly impairs health or psychosocial functioning”. The psychopathology of eating disorders changed across time under the influence of environmental factors, determining the emergence of new phenotypes. Some of these conditions are still under investigation and are not clearly identified as independent diagnostic entities. In this review, the historic evolution of the eating disorder concept up to the recent Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, has been evaluated. We also examined literature supporting the inclusion of new emergent eating behaviors within the eating disorder spectrum, and their relationship with anorexia, autism, and obsessive–compulsive disorder. In particular, we focused on what is known about the symptoms, epidemiology, assessment, and diagnostic boundaries of a new problematic eating pattern called orthorexia nervosa that could be accepted as a new psychological syndrome, as emphasized by an increasing number of scientific articles in the last few years.