Chronic hyperglycemia is believed to play a pivotal role in the development of diabetic complications. It was found that hyperglycemia triggered a number of mechanisms that evoke overproduction of reactive oxygen species (ROS). Diabetes mellitus is associated with an increased level of free radicals, disturbances of the enzymatic antioxidant defense system and lower concentration of exogenous antioxidants. In consequence, these abnormalities lead to a redox imbalance called oxidative stress. The aim of the present study is to summarize the role of reactive oxygen species and changes in the antioxidant defense system in the development of diabetic complications.