The DJ-1 (also called the DJ-1/PfpI, ThiJ/PfpI, or DJ-1/ThiJ/PfpI) superfamily is a structural and functional diverse group of proteins that are present in most organisms. Many of these proteins remain poorly characterized at the biochemical level, but include some known chaperones, proteases, and various stress response proteins that remain mechanistically mysterious. This chapter outlines what is known from a structural perspective about the cellular and biochemical functions of many of these proteins from distinct clades of the superfamily in several organisms. In humans, DJ-1 appears to function primarily as a redox-responsive protein that may act as a sensor for imbalances in cellular redox state. Because mutations in human DJ-1 cause certain types of heritable Parkinson’s disease, the role of oxidative posttranslational modifications and pathogenic mutations in human DJ-1 is emphasized in the latter sections of this chapter.