Reactive oxygen species (ROS), as well as reactive nitrogen species (RNS) play either harmful or beneficial role in biological systems. Beneficial effects of ROS include physiological roles in cellular responses against infectious agents and in several cellular signalling pathways. Harmful effects are due to high concentrations of ROS, which can damage biomolecules, including lipids, proteins and nucleic acids. The harmful effects of ROS are counterbalanced by the antioxidant action of both antioxidant enzymes and non-enzymatic antioxidants; however, despite the presence of the cells antioxidant system, oxidative damage accumulates during the life cycle and has been proposed to play a pivotal role in the development of age-dependent diseases such as atherosclerosis, arthritis, neurodegenerative disorders and cancer. Numerous epidemiological studies indicate that a reduced risk of various lifestyle diseases, mainly cardiovascular diseases and cancer, as well as other disorders, is associated to a diet rich in fruits, vegetables and their products. The drive to enhance the consumption of fruits and vegetables in the human diet is linked with positive effects of beneficial antioxidants impacting on health promotion. In this review we present an outline of main roles of ROS in biological processes and diseases and how natural bioactive compounds of fruits and vegetables determine their health-promoting properties.