Through co-evolution of plants and animals, human organism has adapted to optimize the benefits of plant substances, and hormetically use those which are poisonous, as a protection against oxidative stress. Oxidative stress occurs due to an imbalance between reduction and oxidation reactions in the cell, it is characterized by an overproduction of reactive oxygen species, which indiscriminately devastate lipids, proteins and DNA. Such damages may cause cardiovascular, neurodegenerative diseases and tumors. Plant bioactive compounds due to the chemical structure of their functional groups have an antioxidant effect by participating in enzymatic reactions of the redox system, chelating undesired compounds and metals and by facilitating the intake and stabilization of essential substances. They have an anti-inflammatory effect on the circulatory system, reduce cholesterol levels in the blood and reduce oxidative stress in general. Plant bioactive compounds can also protect nervous tissues, increase synapse plasticity, thus enhancing cognitive functions and memory. They can also chelate carcinogens and have a prooxidative, destructive, effect on tumor cells specifically. It is scientifically proven that a balanced diet can prevent, control and stop the development of diseases through the action of plant bioactive compounds in oxidative stress and inflammation, and also decrease side effects and costs of medical treatments.