Basic research has thrown new light on the importance of free radicals in biol- ogy and medicine. The clarification of interactions opens numerous physio- logical and pathological processes to closer scrutiny. The pathophysiological processes are now understood, even though the basic mechanisms of free radi- cal reaction have as yet escaped full explanation. Asystematic approach makes it possible to re-evaluate and contrast the in- dications, reactions, and prospects for success. The extracellular significance of oxygen radicals in inflammatory condi- tions and their role in the destruction of bacteria in the phagosome of granu- locytes were primary factors intensifying research. Nature also provides a great number of examples which demonstrate that through the emergence of free radicals highly toxic combinations can form from innocuous substances. The reverse principle may also be encountered. Taking into account free radical reactions in pathomorphological investi- gations helps prepare the ground for causal therapy. Research into these phen- omena will contribute to a better understanding of physiological and patho- physiological processes. Many questions still remain unanswered, and it will take several years of research before the final stone is unturned.