This review discusses the role of imaging in photodynamic therapy (PDT), focusing on mechanisms, monitoring, and optimization. PDT uses light-activatable photosensitizers (PS) and light to generate reactive species, causing cytotoxicity. Imaging is used to monitor PDT outcomes and guide treatment. PSs can also serve as fluorescent imaging agents, enabling visualization of tumors. The review covers the history of PDT, the photochemical and photophysical principles underlying PDT, and the use of imaging in PDT for diagnosis, therapy guidance, and monitoring. It discusses various imaging modalities, including fluorescence imaging, and their applications in PDT. The review also highlights the potential of combining imaging with PDT to improve treatment outcomes. The use of PS fluorescence for detecting disease and optimizing surgical resection is discussed, along with the development of new PSs and imaging techniques. The review concludes with future directions in PDT and imaging, emphasizing the importance of integrating imaging with PDT for improved therapeutic strategies.This review discusses the role of imaging in photodynamic therapy (PDT), focusing on mechanisms, monitoring, and optimization. PDT uses light-activatable photosensitizers (PS) and light to generate reactive species, causing cytotoxicity. Imaging is used to monitor PDT outcomes and guide treatment. PSs can also serve as fluorescent imaging agents, enabling visualization of tumors. The review covers the history of PDT, the photochemical and photophysical principles underlying PDT, and the use of imaging in PDT for diagnosis, therapy guidance, and monitoring. It discusses various imaging modalities, including fluorescence imaging, and their applications in PDT. The review also highlights the potential of combining imaging with PDT to improve treatment outcomes. The use of PS fluorescence for detecting disease and optimizing surgical resection is discussed, along with the development of new PSs and imaging techniques. The review concludes with future directions in PDT and imaging, emphasizing the importance of integrating imaging with PDT for improved therapeutic strategies.