Part V focuses on research findings on orthorexia nervosa across two decades. It demonstrates the results of the studies conducted in non-clinical and clinical samples to provide the current knowledge on orthorexia nervosa. It focuses on college and university students and the general population as well as on patients with eating disorders, obsessive-compulsive disorder, somatic symptom disorder and depression. Research on orthorexia nervosa in non-Western societies, with the example of East Asian countries (Mainland China and South Korea), is also presented. A summation of the highlights is included at the end of this chapter. The commentary of the invited international expert (Dr Jinbo He, the Chinese University of Hong Kong, China) provide valuable insights on orthorexia nervosa.