In accordance with the mechanism of suppression of free radical overproduction in biological systems all antioxidants can be divided into two main groups: chain-breaking antioxidants and preventive antioxidants. Chain-breaking antioxidants, often referred to as free radical scavengers, protect against oxidative stress as a result of scavenging initial, peroxyl and rarely alkyl radicals. Preventive antioxidants act as chelators of transition metals, inhibitors of enzymatic systems responsible for the generation of reactive oxygen species (ROS) or reduce hydrogen peroxides and organic hydroperoxides and can prevent an appearance of initiating radical and frustrate a free radical chain reaction from ever setting in motion. Biological and health effects of any given antioxidant depends on numerous factors, such as the chemical reactivity toward radicals or another target related to oxidative stress, absorption and distribution in body tissue. Understanding specific mechanisms by which antioxidants may affect pathogenesis of inflammatory and cardio-vascular diseases, neurological disorders and cancer might create a wealth of potential for the treatment and prevention of human diseases.