Abstract： Human skin is mainly exposed to infrared radiation (IR) from natural sunlight as well as artificial photosources that are increasingly used for cosmetic or medical purposes. Epidemiological data and clinical observations, however, indicate that infrared radiation cannot be considered as totally innocuous to human skin. In response to long-term IR irradiation, ROS formation can be increased with the activation of mitogen-activated protein kinase signaling pathways followed by the upregulation of matrix metalloproteinase-1 expression. Therefore, as a part of natural sunlight, infrared radiation significantly contributes to extrinsic skin aging.