Singlet oxygen (¹O₂) is a highly reactive oxygen species involved in numerous chemical and photochemical reactions in different biological systems and in particular, in photodynamic therapy (PDT). However, the quantification of ¹O₂ generation during in vitro and in vivo photosensitization is still technically challenging. To address this problem, indirect and direct methods for ¹O₂ detection have been intensively studied. This review presents the available methods currently in use or under development for detecting and quantifying ¹O₂ generation during photosensitization. The advantages and limitations of each method will be presented. Moreover, the future trends in developing PDT-¹O₂ dosimetry will be briefly discussed.