The chapter provides a review on singlet oxygen-sensitized delayed fluorescence (SOSDF). The molecular mechanism, kinetics properties, experimental instrumentation, and specific applications of SOSDF are overviewed. Concurrent types of delayed fluorescence that may overlap with SOSDF are also discussed. It provides a framework for distinguishing among different types of delayed fluorescence, which is necessary for correct analysis of the experimental data. SOSDF of a photosensitizer offers an alternative method for singlet oxygen (¹O₂) detection and monitoring employing the photosensitizer itself as a probe. SOSDF allows to determine the rate constants of ¹O₂ formation and deactivation in a time-resolved experiment similarly to direct ¹O₂ near-infrared phosphorescence. However, SOSDF can be orders of magnitude more intense and manifests itself in the visible spectrum, which makes the detection less experimentally demanding. SOSDF has been observed in a wide range of systems, including living cells and tissues incubated with photosensitizers. These properties make SOSDF a promising method for online monitoring during photodynamic therapy treatment.