Olive leaves have always aroused an important interest, especially for folk medicine. Polyphenols contained in olive leaves have played an important role to this end, because they have demonstrated to be responsible for their anti-carcinogenic, anti-inflammatory, and antimicrobial proprieties. Olive leaves have common phenolics with other plants, but they also contain phenolics belonging to the secoiridoids family (exclusive to the Oleaceae family). Chemical, agronomical and medicinal researches have contributed together to highlight the interest in the use of olive leaves as a potential source of phenolic compounds for the production of functional food and nutraceuticals. The aim of this review is to provide a guideline summarizing the great information available about phenolic compounds of olive leaves. Therefore, from one side, it has been reported the availability of leaves as by-products, a brief description of the main phenolics identified in leaves, as well as the main analytical methods used for their extraction and determination. From another side, the effects of abiotic and biotic factors on the phenolic compound content in leaves have also been exposed for the first time, and finally, an overview of the main research studies dealing with the beneficial effects of olive leaves phenolic compounds has been included.