The cause of acne is still obscure, but genetic predisposition, sebaceous overactivity, overgrowth of bacterial flora and exposure to comedogenic substances are all significant factors. Acne lesions occur mainly in sebaceous follicles, which are characterized by deep follicular canals and large sebaceous glands. The associated seborrhea is not due to a circulatory excess of androgens but may be caused by a local amplification of androgenic activity. This, in turn, may be due to large numbers of androgen receptors and a high concentration of enzymes such as 17beta-hydroxysteroid dehydrogenase, within the sebaceous gland itself. Hyperkeratosis of the retention type in the pilary infrainfundibulum obstructs the outflow of sebum and keratin flakes. This favors the proliferation of Propionibacterium acnes which may initiate inflammation in microcomedos and lead to formation of pustules, papules or nodules. Topical therapy with tretinoin, benzoyl peroxide and antibiotics such as clindamycin is widely used today. Oral tetracyclines and other chemotherapeutic agents remain necessary in severe cases.