Sebum, the oily secretion of sebaceous skin glands, constitutes, blended with sweat, the hydrolipidic film which protects the epidermis from outside attacks and drying.

Sebaceous secretions are regulated by many factors. A hormonal imbalance or a disturbance of the balance (as is generally experienced during adolescence or pregnancy), stress, pollution, smoking, poor food choices or overly agressive cosmetic treatments can cause hypersecretion of sebum and lead to oily skin, visibly noticeable or apparent to the touch.