The article by Rafael Radi discusses the occurrence and significance of protein tyrosine nitration under disease conditions, highlighting its shift from the physiological actions of nitric oxide (NO) to potentially pathogenic oxidative pathways. Tyrosine nitration is mediated by reactive nitrogen species such as peroxynitrite anion (ONOO−) and nitrogen dioxide (*NO2*), which are formed as secondary products of NO metabolism in the presence of oxidants like superoxide radicals (O2−), hydrogen peroxide (H2O2), and transition metal centers. The interplay between NO and oxidants, and the identification of the proximal intermediates responsible for nitration *in vivo* have been controversial. While peroxynitrite has been shown to mediate nitration *in vitro*, its role *in vivo* has been questioned, and alternative pathways involving nitrite/H2O2/hemeperoxidase and transition metal-dependent mechanisms have been proposed. A balanced analysis of existing evidence indicates that multiple nitration pathways can contribute to tyrosine nitration *in vivo*, and most pathways involve free radical biochemistry with carbonate radicals (CO3−) and/or oxo–metal complexes oxidizing tyrosine to tyrosyl radical, followed by the diffusion-controlled reaction with *NO2* to yield 3-nitrotyrosine. Although protein tyrosine nitration is a low-yield process *in vivo*, 3-nitrotyrosine has become a relevant biomarker of NO-dependent oxidative stress. Site-specific nitration can modify protein function and promote biological effects. The tissue distribution and quantitation of protein 3-nitrotyrosine, recognition of the predominant nitration pathways, and individual identification of nitrated proteins in disease states open new avenues for understanding and treating human pathologies. The article also discusses the role of iron regulatory proteins (IRPs) in regulating iron homeostasis and their potential involvement in nitration reactions. Finally, the biological significance of protein tyrosine nitration is explored, including its impact on enzyme activity and cell death, and the potential of nitration as a biomarker for disease risk.The article by Rafael Radi discusses the occurrence and significance of protein tyrosine nitration under disease conditions, highlighting its shift from the physiological actions of nitric oxide (NO) to potentially pathogenic oxidative pathways. Tyrosine nitration is mediated by reactive nitrogen species such as peroxynitrite anion (ONOO−) and nitrogen dioxide (*NO2*), which are formed as secondary products of NO metabolism in the presence of oxidants like superoxide radicals (O2−), hydrogen peroxide (H2O2), and transition metal centers. The interplay between NO and oxidants, and the identification of the proximal intermediates responsible for nitration *in vivo* have been controversial. While peroxynitrite has been shown to mediate nitration *in vitro*, its role *in vivo* has been questioned, and alternative pathways involving nitrite/H2O2/hemeperoxidase and transition metal-dependent mechanisms have been proposed. A balanced analysis of existing evidence indicates that multiple nitration pathways can contribute to tyrosine nitration *in vivo*, and most pathways involve free radical biochemistry with carbonate radicals (CO3−) and/or oxo–metal complexes oxidizing tyrosine to tyrosyl radical, followed by the diffusion-controlled reaction with *NO2* to yield 3-nitrotyrosine. Although protein tyrosine nitration is a low-yield process *in vivo*, 3-nitrotyrosine has become a relevant biomarker of NO-dependent oxidative stress. Site-specific nitration can modify protein function and promote biological effects. The tissue distribution and quantitation of protein 3-nitrotyrosine, recognition of the predominant nitration pathways, and individual identification of nitrated proteins in disease states open new avenues for understanding and treating human pathologies. The article also discusses the role of iron regulatory proteins (IRPs) in regulating iron homeostasis and their potential involvement in nitration reactions. Finally, the biological significance of protein tyrosine nitration is explored, including its impact on enzyme activity and cell death, and the potential of nitration as a biomarker for disease risk.