Free Radicals are molecules with an unpaired electron. They are generated in our body by various endogenous systems, exposure to different physicochemical conditions or pathophysiological states. At low levels, free radicals exert beneficial effects while at high concentrations, they generate oxidative stress and damage all cell structures. This review deals with the sources of the free radicals, their beneficial and deleterious effects on cellular activities; it highlights the role of free radicals and oxidative stress in various physiological states like cancer, cardiovascular diseases, Alzheimer’s disease, kidney disease, etc. The harmful effect of free radicals is neutralized by antioxidants, which prevents oxidative damage by reacting with free radicals. Therefore, the best remedy is to increase the intake of natural antioxidant.