The accumulated information concerning the involvement of reactive oxygen species in many clinical disorders and disease states has led to the potential for intervention with antioxidants in these cases. There are currently numerous clinical trials involving administration of antioxidants in a variety of conditions such as coronary heart disease, cataract, cancer and neurodegenerative diseases. At the same time therapeutic trials aimed at preventing and delaying the aging process are also under investigation. Numerous disorders, in childhood, have also been linked to oxidative damage. The aim of this review is to provide an overview of oxidative stress, its mechanisms, targets and damage incurred, as pertaining specifically to clinical disorders during childhood. The defense mechanisms against oxidation; the enzymatic antioxidants and low molecular weight antioxidants are defined and a number of methods commonly used for evaluation of oxidative stress (methods for measurement of lipid and protein oxidation end products and methods for measurement of antioxidant defense capacity) described. Specific diseases related to oxidative stress in infancy and childhood are reviewed and the possible effect of nutritional intake on oxidative stress in the healthy child discussed. Other issues addressed include the ability of oxidative stress, as measured in plasma to reflect intratissue oxidation, the need for a simple laboratory method for characterization of an oxidative stress 'profile', the proposed role of oxidative stress in biological processes pertaining to growth and maturation and possible implications of unrestricted antioxidant supplementation.