Plant antioxidants are composed of a broad variety of different substances like ascorbic acid and tocopherols, polyphenolic compounds, or terpenoids. They perform several important functions in plants and humans (e.g., carotenoids function as accessory pigments for light harvesting and provide photoprotection and pigmentation in plants). Monoterpenes and diterpenes, which are the main components of essential oils, act as allelopathic agents, attractants in plant-plant or plant-pathogen/herbivore interactions or repellants. For humans, carotenoids play an important role for health, carotenoids with provitamin A activity are important for vision; other carotenoids influence the human immune function and gap-junctional communication (GJC). Additionally, their antioxidative capacity is believed to be responsible for the health promoting properties of fruits and vegetables. Three main ways of antioxidant action of carotenoids have been detected until now (i.e., quenching of singlet oxygen, hydrogen transfer, or electron transfer). These mechanisms and investigation of antioxidant activity in vitro are discussed in detail. The monoterpenes limonene and perillyl alcohol may be promising substances in cancer therapy. Several investigations have studied the antioxidant activity of monoterpenes and diterpenes or essential oils in vitro. Results as well as the action of a newly discovered, very effective antioxidant (i.e., gamma-terpinene) are discussed. An important point when assessing the antioxidant activity of plant antioxidants is to consider their interaction with other antioxidants. Especially combinations of hydrophilic and lipophilic antioxidants may exert synergistic effects, as has been shown for rutin in combination with gamma-terpinene, lutein, or lycopene.