Reactive oxygen and nitrogen species (ROS and RNS respectively) play an important role in the proper functioning of many cellular processes. Generation of reactive oxygen species is an integral part of aerobic metabolism of cells. Their overproduction and subsequent oxidative stress occurs during pathogenesis of many diseases. Nitrosative stress is very closely linked to oxidative stress. Nitric oxide (NO), can react with molecular oxygen, superoxide anions and metal cations generating consecutive reactive oxygen species. These highly reactive chemical compounds react with proteins impairing their function by oxidation, or nitrosylation of amino acid residues, which may induce apoptosis. Furthermore, nitric oxide enhances the effect induced by cyclooxygenases and becomes a mediator of the inflammatory response. This paper gathers key information on the reactive oxygen and nitrogen species as well as processes in which they participate.