Plants have evolved antioxidant molecules to help them withstand environmental stresses. Humans may also benefit from these defense molecules through their consumption in fruits and vegetables. Dietary antioxidants are indeed believed to play a very important role in the human body defense system, protecting, as in plants, against oxidative damage induced by Reactive Oxygen Species (ROS), which are known to be involved in the pathogenesis of aging and many degenerative diseases such as cardiovascular diseases and cancers. In this review, we compare the systems involved in ROS production and scavenging in humans and in plants. We focus mainly on the description of the best-known dietary antioxidants: ascorbate (vitamin C), tocopherols (vitamin E), carotenoids, and phenolic compounds. Their redox properties, metabolism and functions are discussed from both a human and a plant perspective.