Demodex infestation is a common cause of blepharitis and now is drawing more and more attention from clinicians. Demodex mite is the most common age-related ectoparasite of human being and may be detected from asymptomatic normal population. Furthermore, the pathogenesis of demodicosis remains unclear. Therefore, the commonly accepted diagnostic criteria of demodex infestation related blepharitis have not been established. Herein, we summarize the related studies about ocular demodicosis and comment the debates about the diagnosis of the disease. We also propose diagnostic criteria of demodex blepharitis. (Chin J Ophthalmol, 2017, 53: 648-652).