Oxidative stress, an excess of radicals and other reactive oxygen species (ROS) relative to antioxidant defence, has been suggested as a major disease mechanism from single cell to multicellular organisms. With the idea to overcome this, antioxidants are heavily marketed, yet without proof of their effectiveness. Rather, evidence suggests adverse effects. This paradox is due to the fact that ROS are not only ‘bad’, but – in tightly regulated amounts – also act as essential signalling molecules and are essential for host defence. In a tightly controlled fashion ROS regulate for example proliferation, differentiation, metabolic actions, apoptosis and angiogenesis. Moreover, a deficiency in ROS can result in similarly harmful reductive stress. Thus, the roles of ROS in physiology and pathophysiology need to be completely re-defined and much better understood before safe applications are possible. Unravelling the fine balance between ROS acting as a friend or a foe is fundamental to understand aerobic life. To advance this important area of biology and medicine, highly synergistic approaches combining diverse and scattered disciplines are needed. EU-ROS brings together multi-disciplinary experts to enhance the competitiveness of European research. By applying fundamentally new approaches EU-ROS generates advanced knowledge and translate this into novel applications ranging from medicine to crop science. This COST Action supports and seeks participation of talented students and early career researchers.