The free radical nitric oxide (NO) is a key mediator in different physiological processes such as vasodilation, neurotransmission, inflammation, and cellular immune responses, and thus preserving its bioavailability is essential. In several disease conditions, superoxide radical (O) production increases and leads to the rapid "inactivation" of NO by a diffusion-controlled radical termination reaction that yields a potent and short-lived oxidant, peroxynitrite. This reaction not only limits NO bioavailability for physiological signal transduction but also can divert and switch the biochemistry of NO toward nitrooxidative processes. Indeed, since the early 1990s peroxynitrite (and its secondary derived species) has been linked to the establishment and progression of different acute and chronic human diseases and also to the normal aging process. Here, we revisit an earlier and classical review on the role of peroxynitrite in human physiology and pathology (Pacher P, Beckman J, Liaudet L. 87: 315-424, 2007) and further integrate, update, and interpret the accumulated evidence over 30 years of research. Innovative tools and approaches for the detection, quantitation, and sub- or extracellular mapping of peroxynitrite and its secondary products (e.g., protein 3-nitrotyrosine) have allowed us to unambiguously connect the complex biochemistry of peroxynitrite with numerous biological outcomes at the physiological and pathological levels. Furthermore, our current knowledge of the NO/O and peroxynitrite interplay at the cell, tissue, and organ levels is assisting in the discovery of therapeutic interventions for a variety of human diseases.