This book describes studies on antioxidant activities of a number of chemical compounds, natural in origin or synthesized in the laboratory. This study results identification of a total of eighty four compounds as antioxidants. Out of these eighteen new and twelve known phytochemicals were isolated from different medicinally important plants and fungal cultures, while fifty four synthetic chemical constituents, belonging to six different classes of compounds, were identified as antioxidants. All the compounds were checked for radical scavenging activity in in vitro methods. The compounds were also assayed for xanthine oxidase inhibition. Based on the results, structure- activity relationship studies on different classes of antiradical and xanthine oxidase inhibitors were conducted. The six most active xanthine oxidase inhibitors were also studied for their kinetic behavior. The most active compounds were subjected to in vivo studies using CCl4-induced liver toxicity assay in rat model . Subsequently, these compounds were also checked for their cytotoxicity using a sensitive in vitro cytotoxicity assay on human neutrophils.