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Ultraviolet photons can be absorbed by various photosensitizers ��endogenous and exogenous�� and chromophores in skin, thereafter stimulate the formation of reactive oxygen species ��ROS�� through electronic transfer, which are involved in many pathological processes in skin. Although ROS are partly removed by nonenzymic and enzymic antioxidants, residual ROS can cause a pro-oxidant/antioxidant disequilibrium defined as oxidative stress that subsequently leads to a change in skin pigmentation, damage to such molecules as DNA, as well as an increment in the expression of matrix metalloproteinases, cyclooxygenases, nitric oxide synthase, heme oxygenase, and so on.