Reactive oxygen species (ROS) play a crucial role in plant cell signaling and stress responses. Under normal conditions, ROS are produced at low levels and serve as signaling molecules involved in various physiological processes. However, during stress conditions, the balance between ROS production and detoxification is disrupted, leading to an excessive accumulation of ROS in plant cells. This chapter provides an overview of the generation and consequences of ROS in plant cells during stress conditions. We discuss the sources of ROS production, including both enzymatic and nonenzymatic processes, and the factors that contribute to their accumulation. Furthermore, we explore the impact of ROS on plant cells, including oxidative damage to cellular components, activation of defense mechanisms, and modulation of gene expression. Understanding the generation and consequences of ROS during stress conditions is crucial for developing strategies to enhance plant stress tolerance and improve crop productivity.