Oxidative stress is a host defense mechanism whose involvement in maintaining homeostasis and/or inducing disease has been widely investigated over the past decade. Various reactive oxygen species (ROS) have been defined and the enzymes involved in generating and/or eliminating them have been widely studied. In this review we briefly discuss general mechanisms of oxidative stress and the oxidative stress response of the host. We focus primarily on hydrogen peroxide and summarize the systems involved in its formation and elimination. We describe mechanisms whereby hydrogen peroxide and other ROS can modify protein conformation and, thus, alter protein function, and describe a group of transcription factors whose biological activity is modulated by the redox state of cells. These basic aspects of oxidative stress are followed by a discussion of mechanisms whereby hydrogen peroxide and other ROS can modulate some physiological and pathological processes, with special emphasis on wound healing and scarring of the liver.