Reactive oxygen species (ROS) have long been viewed as harmful byproducts of cellular metabolism, but recent evidence suggests they play crucial roles in regulating intracellular signaling pathways. ROS modulate the activity of target proteins through covalent modification of specific cysteine residues, leading to reversible changes in enzymatic function. These modifications can influence a wide range of physiological processes, from growth factor response to inflammatory responses, and dysregulation of ROS signaling may contribute to various diseases. ROS are produced by various cellular sources, including NADPH-dependent oxidases and mitochondria, and their production is tightly regulated. The accumulation of ROS near specific targets can be achieved through colocalization with the source of ROS, such as through aquaporin channels. Antioxidant proteins, like thioredoxin and peroxiredoxins, not only scavenge ROS but also participate in redox signaling by modulating the activity of target molecules. The balance between ROS production and antioxidant defenses is critical for maintaining cellular redox homeostasis, and disruptions in this balance have been linked to insulin resistance, aging, and other pathologies. Understanding the complex interplay between ROS and redox signaling is essential for comprehending the physiological and pathological states of cells.Reactive oxygen species (ROS) have long been viewed as harmful byproducts of cellular metabolism, but recent evidence suggests they play crucial roles in regulating intracellular signaling pathways. ROS modulate the activity of target proteins through covalent modification of specific cysteine residues, leading to reversible changes in enzymatic function. These modifications can influence a wide range of physiological processes, from growth factor response to inflammatory responses, and dysregulation of ROS signaling may contribute to various diseases. ROS are produced by various cellular sources, including NADPH-dependent oxidases and mitochondria, and their production is tightly regulated. The accumulation of ROS near specific targets can be achieved through colocalization with the source of ROS, such as through aquaporin channels. Antioxidant proteins, like thioredoxin and peroxiredoxins, not only scavenge ROS but also participate in redox signaling by modulating the activity of target molecules. The balance between ROS production and antioxidant defenses is critical for maintaining cellular redox homeostasis, and disruptions in this balance have been linked to insulin resistance, aging, and other pathologies. Understanding the complex interplay between ROS and redox signaling is essential for comprehending the physiological and pathological states of cells.