Reactive oxygen species (ROS), are generated during normal physiological processes. ROS are toxic and oxidize of various cell constituents such as DNA, lipids and proteins. The oxidation products so produced may cause damage to cellular machinery, ultimately leading to cell death. ROS have been implicated in a myriad of diseases such as various forms of cancer, atherosclerosis, ischemic reperfusion injury, neurodegenerative diseases, and chronic inflammatory diseases, such as rheumatoid and psoriatic arthritis. Tumor promoters, such as phorbol-12-myristate-13-acetate (PMA) enhance the generation of these ROS, through protein kinase C pathway, to activate NADPH oxidase and xanthine oxidase. Nitric oxide (NO) plays an important role in inflammation and in the multiple stages of carcinogenesis. The suppressive effect of polyphenols on ROS production, monitored by flow cytometry using dichlorodihydrofluorescein diacetate (DCFH-DA) and dihydroethidium (DHE), and NO generation are described.