For more than two decades since the description of orthorexia nervosa, its prevalence has been investigated in various subgroups and populations worldwide using different measurement tools and the value of cut-off points (See Chapter 6). Due to the limitations regarding the lack of reliable and valid assessment tools as well as the absence of detailed diagnostic criteria, the assessment of the prevalence of orthorexia nervosa presents a difficult challenge. The prevalence of orthorexia nervosa depends on the study group, country and the instrument being used. In the previous literature review (Niedzielski & Kaźmierczak-Wojtaś, 2021), the prevalence rate of orthorexia nervosa has been reported to range from 6.9% to 90.6% [sic] using ORTO-15. The recent systematic review and meta-analysis, including 30,476 individuals from 18 countries (López-Gil et al., 2023), demonstrated that the overall proportion of orthorexia nervosa symptoms (assessed using the ORTO-15 with a cut-off of <35) was 27.5%.