The Mediterranean olive tree, Olea europaea, defends itself against bacteria, fungi, parasites and insects through the eutrophic compounds it produces. Its leaf extract, popular in herbal and folk remedies, has been widely studied in recent years for its microbial balancing properties andcardiometabolic health benefits. Consistent with its traditional use, olive leaf extracts have been shown to support microbial balance without suppressing immune system function. A well-researched bioactive constituent is oleuropein, a unique phenolic glucoside (structurally classified as an iridoid). Oleuropein is metabolized by intestinal bacteria to hydroxytyrosol, a phenolic antioxidant that is hypothesized to contribute to the clinical benefits of olive oil and leaf extracts. Flavonoids, such as rutin, luteolin and hesperidin, may alsocontribute to its functional profile.