Reactive oxygen species (ROS) play a crucial role in host defense against pathogens by exerting antimicrobial activity. ROS, primarily formed from the partial reduction of molecular oxygen, include superoxide (O$_2$^-), hydrogen peroxide (H$_2$O$_2$), hydroxyl radical (*OH), and singlet oxygen (1O$_2$). These ROS have different kinetics and levels of activity, with O$_2$^- and H$_2$O$_2$ being less acute compared to *OH and 1O$_2$. *OH and 1O$_2$ are highly toxic and lethal due to their inability to be detoxified by endogenous antioxidants. The review highlights methods of ROS formation, their mechanisms of action, and their role in antimicrobial host defense systems. Antimicrobial approaches utilizing ROS include bactericidal antibiotics, photodynamic therapy, titanium dioxide photocatalysis, cold plasma, and medicinal honey. The review also covers reactive nitrogen species and related therapeutics, such as acidified nitrite and nitric oxide-releasing nanoparticles. The article discusses the production and interconversion of ROS, their roles in cellular signaling, and their impact on various diseases. It explores the antioxidant defense mechanisms in living organisms, including vitamins and membrane defenses, as well as the genetic responses to ROS and oxidative stress in microorganisms. The mammalian innate immune system's mechanisms of recognizing and killing foreign invaders using ROS are also discussed.Reactive oxygen species (ROS) play a crucial role in host defense against pathogens by exerting antimicrobial activity. ROS, primarily formed from the partial reduction of molecular oxygen, include superoxide (O$_2$^-), hydrogen peroxide (H$_2$O$_2$), hydroxyl radical (*OH), and singlet oxygen (1O$_2$). These ROS have different kinetics and levels of activity, with O$_2$^- and H$_2$O$_2$ being less acute compared to *OH and 1O$_2$. *OH and 1O$_2$ are highly toxic and lethal due to their inability to be detoxified by endogenous antioxidants. The review highlights methods of ROS formation, their mechanisms of action, and their role in antimicrobial host defense systems. Antimicrobial approaches utilizing ROS include bactericidal antibiotics, photodynamic therapy, titanium dioxide photocatalysis, cold plasma, and medicinal honey. The review also covers reactive nitrogen species and related therapeutics, such as acidified nitrite and nitric oxide-releasing nanoparticles. The article discusses the production and interconversion of ROS, their roles in cellular signaling, and their impact on various diseases. It explores the antioxidant defense mechanisms in living organisms, including vitamins and membrane defenses, as well as the genetic responses to ROS and oxidative stress in microorganisms. The mammalian innate immune system's mechanisms of recognizing and killing foreign invaders using ROS are also discussed.