In the past decade, a new family of highly conserved antioxidant enzymes, Peroxiredoxins (Prxs), have been discovered and defined. There are two major Prx subfamilies: one subfamily uses two conserved cysteines (2-Cys) and the other uses 1-Cys to scavenge reactive oxygen species (ROS). This review focuses on the four mammalian 2-Cys members (Prx I-IV) that utilize thioredoxin as the electron donor for antioxidation. The array of biological activities of these proteins suggests that they may be evolutionarily important for cell function. For example, Prxs are capable of protecting cells from ROS insult and regulating the signal transduction pathways that utilize c-Abl, caspases, nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1) to influence cell growth and apoptosis. Prxs are also essential for red blood cell (RBC) differentiation and are capable of inhibiting human immunodeficiency virus (HIV) infection and organ transplant rejection. Distribution patterns indicate that Prxs are highly expressed in the tissues and cells at risk for diseases related to ROS toxicity, such as Alzheimer's and Parkinson's diseases and atherosclerosis. This interesting correlation suggests that Prxs are protective against ROS toxicity, yet overwhelmed by oxidative stress in some cells. Prxs tend to form large aggregates at high concentrations, a feature that may interfere with their normal protective function or may even render them cytotoxic. Imbalance in the expression of subtypes can also potentially increase their susceptibility to oxidative stress. Understanding the function and biological role of Prxs may lead to important discoveries about the cellular dysfunction of ROS-related diseases ranging from atherosclerosis to cancer to neurodegenerative diseases.