Singlet oxygen (¹O₂) is molecular oxygen in the excited state with high energy and electrophilic properties. It is widely found in nature, and its important role is gradually extending from chemical syntheses and medical techniques to environmental remediation. However, there exist ambiguities and controversies regarding detection methods, generation pathways, and reaction mechanisms which have hindered the understanding and applications of ¹O₂. For example, the inaccurate detection of ¹O₂ has led to an overestimation of its role in pollutant degradation. The difficulty in detecting multiple intermediate species obscures the mechanism of ¹O₂ production. The applications of ¹O₂ in environmental remediation have also not been comprehensively commented on. To fill these knowledge gaps, this paper systematically discussed the properties and generation of ¹O₂, reviewed the state-of-the-art detection methods for ¹O₂ and long-standing controversies in the catalytic systems. Future opportunities and challenges were also discussed regarding the applications of ¹O₂ in the degradation of pollutants dissolved in water and volatilized in the atmosphere, the disinfection of drinking water, the gas/solid sterilization, and the self-cleaning of filter membranes. This review is expected to provide a better understanding of ¹O₂-based advanced oxidation processes and practical applications in the environmental protection of ¹O₂.