Metabolic activation of molecular oxygen frequently gives rise to reactive oxygen species (ROS), which may be toxic to biological systems. Cells have evolved antioxidant protection mechanisms, including enzymes and low molecular weight molecules. Cellular antioxidative capacity may be overwhelmed by a flux of ROS, resulting in oxidative stress to cells and tissues. Oxidative stress was shown to be involved in most human pathologies, including cardiovascular diseases, cancer and neurodegenerative diseases. However, ROS also play a central role in various essential cellular processes such as apoptosis, which is the physiological suicide pathway. Our laboratory is involved in basic and applied-oriented research on the deleterious and beneficial effects of ROS in human pathologies and environmental stresses encountered by lichens and bacteria.