Ocular demodicosis is a series of ocular surface diseases caused by demodex infestation. Demodex is the most common ectoparasite living on humans, only two species have been identified on humans, namely Demodex folliculorum and Demodex brevis. Demodex has highly complex biological characteristics. The pathogenic characteristics of mites, such as internal and external structure, living habits, are one of the basic factors that can lead to diseases. Differential expression of important functional genes is likely correlated with pathogenicity disparities between these two demodex species. Both demodex mites can induce histopathological changes, such as follicular distention, epithelial hyperplasia, reactive hyperkeratinization, and granulomatous reaction. Microorganisms might be involved in the pathogenesis of Demodex, Bacillus oleronius and Corynebacterium kroppenstedtii subsp. are reportedto be symbiotic bacteria inside demodex mites. In addition, the mites may act as vectors by carrying bacteria on their surface. Demodex might have the capacity of immune-modulating, due to immunosuppression and risk factors, and increased demodex mite numbers may trigger a host immune reaction by activation of multiple pathways leading to ocular surface inflammatory changes. In order to perform deep research on the pathogenesis of demodex, the present review would provide insight into ocular demodicosis, which will ultimately improve the diagnosis and treatment of this disease.(Int Rev Ophthalmol, 2021, 45: 431-436)