Oleacein is the second most important phenolic constituent of olive oil and, similar to oleocanthal, its concentration is varying in deferent olive oils depending in genetic and pedoclimatic factors. Oleacein is formed after the enzymatic transformation of oleuropein, which is the most abundant phenolic of the olive fruit, and which has numerous of biological activities. Oleacein, due to the dihydroxy phenolic ring in its structure, was initially considered as a simple antioxidant. After well documented studies though, which are presented below, many biological activities have been reveal, which partly explain the health protective properties of olive oil.