In some cases, detrimental consequences on health are generated by self-imposed dietary rules intended to promote health. The pursuit of an "extreme dietary purity" due to an exaggerated focus on food may lead to a disordered eating behavior called "orthorexia nervosa" (ON). ON raises a growing interest, but at present there is no universally shared definition of ON, the diagnostic criteria are under debate, and the psychometric instruments used in the literature revealed some flaws. This narrative review of the literature aims at assessing state of the art in ON definition, diagnostic criteria and related psychometric instruments and provides research propositions and framework for future analysis.