Obagi Professional-C 20% is a powerful, potent antioxidant that is responsible for protecting the skin from break down due to free radicals in the environment. Vitamin C is the only know anti-oxidant to stimulate collagen synthesis given a more youthful, plump appearance to the skin. While using the Obagi Professional-C Serums you will notice an overall brighter, more even toned rejuventated looking skin.