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Reactive oxygen species (ROS) are molecules capable of independent existence, containing at least one oxygen atom and one or more unpaired electrons. This group includes oxygen free radicals, e.g. superoxide anion radical, hydroxyl radical, hydroperoxyl radical, singlet oxygen, as well as free nitrogen radicals. Under physiological conditions, small quantities of ROS are formed during cell processes, such as aerobic respiration or inflammatory processes, mainly in hepatocytes and macrophages. Reactive oxygen species are primarily signalling molecules. In addition, they induce cell differentiation and apoptosis, thus contributing to the natural ageing process. They also participate in muscle contractions, regulation of vascular tone, and determine bactericidal and bacteriostatic activity. Increased production of free radicals is caused by excessive exposure to UV radiation, long-term stress conditions, intense physical exercise, improper diet and use of stimulants. Under physiological conditions, there is a balance between the generation and removal of free radicals from the body. The aim of the article was to review the current state of knowledge regarding oxidative stress, free radical function and free radical diseases. The search was performed using search engines such as PubMed and Google Scholar. The keywords used in the search included: oxygen radicals, oxidative stress, free radical-related diseases. Excessive formation of free radicals contributes to oxidative stress, causing damage at the molecular and cellular level. Reactive oxygen species in vitro cause chemical modifications as well as damaging effects to proteins (aggregation, denaturation), lipids (peroxidation), carbohydrates and nucleotides (changes in the DNA structure). These changes contribute to the development of many free radical-mediated diseases. Oxidative stress has a particularly adverse effect on the circulatory, respiratory and nervous systems.