SEN, C. K. Antioxidant and redox regulation of cellular signaling: introduction. Med. Sci. Sports Exerc., Vol. 33, No. 3, 2001, pp. 368–370. Oxidation-reduction (redox) based regulation of gene expression appears to be a fundamental regulatory mechanism in cell biology. This basic information has been exploited to develop novel strategies in clinical therapeutics. In contrast to the conventional idea that reactive species mostly serve as a trigger for oxidative damage of biological structures, we now know that low physiologically relevant concentration of reactive oxygen species can regulate a variety of key molecular mechanisms. Physical exercise causes redox changes in various cells and tissues. The molecular implications of such change are yet uncharacterized. The five component articles of this symposium discuss skeletal muscle contraction, cell adhesion, heat shock proteins, programmed cell death, and carbohydrate metabolism as they relate to physical exercise.