The discovery of nitric oxide (NO) in mammalian cells has sparked extensive research in biology and medicine. NO, initially identified as an endothelial-derived relaxing factor, is now recognized as a fundamental signaling molecule regulating various cellular functions and a potent mediator of cellular damage. The cytotoxicity attributed to NO is largely due to peroxynitrite, formed from the reaction between NO and superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins, leading to oxidative injuries ranging from subtle cell signaling modulations to severe necrosis or apoptosis. In vivo, peroxynitrite generation is a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. The formation of peroxynitrite is inevitable due to the diffusion-limited reaction between NO and superoxide, and its biological actions are more subtle and specific compared to hydroxyl radical. The review discusses the historical context, biological chemistry, and therapeutic potential of NO and peroxynitrite in various diseases.The discovery of nitric oxide (NO) in mammalian cells has sparked extensive research in biology and medicine. NO, initially identified as an endothelial-derived relaxing factor, is now recognized as a fundamental signaling molecule regulating various cellular functions and a potent mediator of cellular damage. The cytotoxicity attributed to NO is largely due to peroxynitrite, formed from the reaction between NO and superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins, leading to oxidative injuries ranging from subtle cell signaling modulations to severe necrosis or apoptosis. In vivo, peroxynitrite generation is a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. The formation of peroxynitrite is inevitable due to the diffusion-limited reaction between NO and superoxide, and its biological actions are more subtle and specific compared to hydroxyl radical. The review discusses the historical context, biological chemistry, and therapeutic potential of NO and peroxynitrite in various diseases.