Acne is easy to diagnose, but its physiopathology is complex and not yet fully elucidated. However, the three factors that are necessary to the genesis of acne are well defined, being seborrhoea, obstruction of the hair follicle and the sebaceous gland, and inflammation of the follicle. To this must be added a genetic predisposition. While the skin lesions leave little doubt about the diagnosis, a thorough examination is needed to evaluate the part due to sebum retention (comedos, microcysts) and that due to inflammatory elements (papules, pustules, nodules), for the lesions are so polymorphous that it would be better to speak of acnes rather than acne. A good knowledge of clinical and physiopathological data leads to the appropriate treatment, since almost all therapeutic failures result from a misappreciation of acne.