This book provides an elucidative account on the functioning and mechanism of antioxidant enzymes. Free radicals are continually generated in living cells and discarded by antioxidant defenses. Antioxidant enzymes are the primary defense against free radicals in animal and plant cells. Uncontrolled generation of reactive oxygen species (ROS) is responsible for causing several human diseases, including diabetes and cancer, because of disturbance in cellular and molecular processes including cell growth, differentiation and proliferation. When cells are introduced to oxidative stress, a defense system supports the expression and regulation of number of antioxidant enzymes as a defense mechanism to secure them from the damage inflicted by free radicals. On the basis of this information, this book has been compiled to overview the significance of antioxidant enzymes in human and plant cells against toxic free radicals, their relationship with various pathophysiologic processes and their probable therapeutic applications.