This review summarizes recent advances in understanding the structure, function, and inhibition of nitric oxide synthases (NOSs) over the past seven years. NOSs are enzymes that catalyze the production of nitric oxide (NO) from L-arginine, using molecular oxygen and NADPH. The enzyme consists of two domains: an oxygenase domain, which binds substrates and cofactors, and a reductase domain, which donates electrons. NOSs exist in three isoforms: neuronal (nNOS), inducible (iNOS), and endothelial (eNOS), each with distinct functions and regulatory mechanisms. The structure of NOS has been elucidated at various levels, including the crystal structures of the oxygenase domains of iNOS and eNOS. These structures reveal the binding sites for key cofactors such as heme, BH4, FAD, and FMN. The enzyme's reaction mechanism remains a topic of debate, with ongoing discussions about the exact nature of the reaction, its mechanism, and the role of the BH4 cofactor. Recent studies suggest that BH4 may play a redox role in the reaction, facilitating electron transfer between the heme and flavin domains. The dimerization of NOS is crucial for its function, with the dimer interface playing a key role in stabilizing the enzyme and facilitating substrate binding. The presence of BH4 and L-arginine promotes and stabilizes the active dimeric form of all three NOS isoforms. However, the exact role of BH4 in the reaction mechanism is still under investigation. The reaction catalyzed by NOS involves the oxidation of L-arginine to citrulline and the production of NO. However, there is ongoing debate about whether NO or nitroxyl (NO⁻) is the primary product of the reaction. This has important implications for understanding the physiological and pathophysiological roles of NOS and its inhibitors. The stoichiometry of the NOS reaction is also a subject of debate, with different models proposed for the consumption of NADPH and the production of NO or nitroxyl. The reaction involves two steps: the first step (monooxygenase I) and the second step (monooxygenase II). The exact mechanism of these steps and the role of the BH4 cofactor in the reaction are still being investigated. The flavin domains of NOS play a crucial role in electron transfer, with FAD and FMN accepting electrons from NADPH and passing them to the heme domain. The heme domain is responsible for the redox reactions involved in the production of NO. The pterin cofactor BH4 is essential for the function of NOS, but its exact role in the reaction mechanism remains unclear. The reaction mechanism of NOS has been compared to that of cytochrome P450 enzymes, with similarities in theThis review summarizes recent advances in understanding the structure, function, and inhibition of nitric oxide synthases (NOSs) over the past seven years. NOSs are enzymes that catalyze the production of nitric oxide (NO) from L-arginine, using molecular oxygen and NADPH. The enzyme consists of two domains: an oxygenase domain, which binds substrates and cofactors, and a reductase domain, which donates electrons. NOSs exist in three isoforms: neuronal (nNOS), inducible (iNOS), and endothelial (eNOS), each with distinct functions and regulatory mechanisms. The structure of NOS has been elucidated at various levels, including the crystal structures of the oxygenase domains of iNOS and eNOS. These structures reveal the binding sites for key cofactors such as heme, BH4, FAD, and FMN. The enzyme's reaction mechanism remains a topic of debate, with ongoing discussions about the exact nature of the reaction, its mechanism, and the role of the BH4 cofactor. Recent studies suggest that BH4 may play a redox role in the reaction, facilitating electron transfer between the heme and flavin domains. The dimerization of NOS is crucial for its function, with the dimer interface playing a key role in stabilizing the enzyme and facilitating substrate binding. The presence of BH4 and L-arginine promotes and stabilizes the active dimeric form of all three NOS isoforms. However, the exact role of BH4 in the reaction mechanism is still under investigation. The reaction catalyzed by NOS involves the oxidation of L-arginine to citrulline and the production of NO. However, there is ongoing debate about whether NO or nitroxyl (NO⁻) is the primary product of the reaction. This has important implications for understanding the physiological and pathophysiological roles of NOS and its inhibitors. The stoichiometry of the NOS reaction is also a subject of debate, with different models proposed for the consumption of NADPH and the production of NO or nitroxyl. The reaction involves two steps: the first step (monooxygenase I) and the second step (monooxygenase II). The exact mechanism of these steps and the role of the BH4 cofactor in the reaction are still being investigated. The flavin domains of NOS play a crucial role in electron transfer, with FAD and FMN accepting electrons from NADPH and passing them to the heme domain. The heme domain is responsible for the redox reactions involved in the production of NO. The pterin cofactor BH4 is essential for the function of NOS, but its exact role in the reaction mechanism remains unclear. The reaction mechanism of NOS has been compared to that of cytochrome P450 enzymes, with similarities in the