Reactive oxygen species (ROS) are essential yet potentially harmful molecules generated as byproducts of various metabolic processes in plants. Under normal physiological conditions, ROS are primarily produced in chloroplasts, mitochondria, and peroxisomes through photosynthetic and respiratory electron transport, photorespiration, and enzymatic reactions. These ROS function as key signaling molecules, regulating growth, development, and stress responses. However, under abiotic (e.g., drought, salinity, extreme temperatures, heavy metals) and biotic (e.g., pathogen attack) stress conditions, ROS production escalates due to enhanced activity of oxidases (e.g., NADPH oxidases, peroxidases) and disruptions in metabolic pathways. While controlled ROS levels mediate defense signaling, excessive accumulation leads to oxidative damage, necessitating a robust antioxidant system for detoxification. This chapter explores the major sources of ROS in plants, their roles in cellular signaling under normal growth conditions, and their overproduction during stress. Understanding these mechanisms is critical for improving plant stress tolerance and developing strategies to enhance crop resilience in a changing climate.