Data from a variety of experimental studies suggesting benefits of the antioxidant supplementation were confronted with those from clinical trials and large metaanalyses which have failed to prove those benefits. Major directions of the biological effects of reactive oxygen species (ROS) and antioxidants were specified. In particular, plausible mechanisms through antioxidants may disturb physiological functions were discussed. The regulatory aspects of ROS potency, and the ways antioxidants may disturb homeostasis were taken particularly into account. Directions of prospective research towards practical use of antioxidants were suggested.