During the past 25 years, the field of free radical biology has germinated, sprouted, and flowered. Free radicals derived frommolecul ar oxygen, formerly of interest only to radiation chemists, are now known to play multiple roles in living systems. We will here consider the generalities of this field with some special focus on skin as a site of oxygen radical production and as a target upon which the damaging propensities of these radicals are exerted.