Reactive oxygen species (ROS) are generated and exploited to regulate many physiological processes, but over-production of ROS may cause or accelerate a diverse array of pathological conditions. To understand precisely how ROS play their biological roles, well-established fluorescence imaging approaches have been successfully used to monitor ROS in living cells. This chapter summarizes recent progress in the design and synthesis of various fluorescent probes for the fluorescence imaging of cellular ROS, including superoxide anion, hydrogen peroxide, hydroxyl radical, lipid peroxy radical, HClO, HOBr, ¹O₂ and O₃.