Olive oil is a natural juice obtained by mechanical or physical means from the fruit of the tree Olea europaea L. Many historians believe that the olive cultivation began during the Palaeolithic and Neolithic periods (5000–3500 bce). Most of the world's olive trees grow in the Mediterranean Basin. Olive oil has always been an important component in the diet of Mediterranean people, but in the past years this oil has become more popular among consumers in Northern Europe, the United States, Canada, and Australia. Olive oil quality depends mainly on the cultivar, microclimate, soil conditions, fruit health, and processing conditions. Several studies have confirmed that extra virgin olive oil (EVOO) helps to reduce oxidative stress, high blood pressure, obesity, fatty liver, and insulin resistance, diseases that are closely related to diabetes and coronary heart disease. This chapter focuses on the origin and history of olive tree, statistics of world production, and consumption of olive oil. It also deals with olive fruit processing, olive oil classification and categories, quality and purity criteria, olive oil components, olive oil analysis, deterioration, and adulteration. Phenolics and other minor functional compounds of olive oil are discussed in detail.