The thesis explores the correlation between oxidative stress, reactive oxygen species and the occurrence of various chronic illnesses and their pathomechanisms on a molecular level. The diseases which are stated are world leading causes of death such as, cardiovascular diseases, cancer, neurodegenerative conditions such as Alzheimer’s disease and diabetes mellitus. The pathways which oxidative stress can induce or exacerbate the aforementioned conditions is explained and analyzed. The positive influence of antioxidants and the possibility of antioxidant therapy and its clinical approach is also discussed.