This review provides an overview of the role of imaging in photodynamic therapy (PDT), a photochemistry-based approach that uses a photosensitizer (PS) and light to generate cytotoxic reactive molecular species. The authors begin by discussing the historical context of PDT, highlighting its ancient origins and early applications. They then delve into the fundamental photochemical and photophysical principles underlying PDT, emphasizing the importance of PS fluorescence for both diagnosis and therapy optimization. The review covers various imaging modalities used in conjunction with PDT, including spectroscopy, microscopy, endoscopy, and tomography. Each section focuses on specific applications of imaging in PDT, such as tumor margin resection, treatment monitoring, and dosimetry. The authors also discuss the potential of combining multiple imaging modalities and contrast agents to enhance therapeutic outcomes. A significant portion of the review is dedicated to the use of PS fluorescence detection (PFD) for disease detection and surgical guidance. Early studies on PS fluorescence, particularly with porphyrins like hematoporphyrin derivative (HpD), laid the groundwork for more advanced applications. The introduction of 5-aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX) as a photosensitizer has improved tumor selectivity and imaging performance. The review highlights the clinical impact of PFD in detecting and guiding resection of bladder cancer, brain cancer, and ovarian cancer. For bladder cancer, ALA-induced PpIX has shown high sensitivity and specificity in detecting cancerous lesions, leading to improved recurrence rates. In brain cancer, PFD has enabled more precise tumor resection, improving progression-free survival. For ovarian cancer, PFD has been promising in early detection and monitoring treatment responses. The authors conclude by discussing future directions, emphasizing the need for further research in multiplexing imaging modalities and developing online monitoring techniques to enhance the effectiveness of PDT. They also highlight the potential of combining imaging with other therapeutic approaches to improve treatment outcomes.This review provides an overview of the role of imaging in photodynamic therapy (PDT), a photochemistry-based approach that uses a photosensitizer (PS) and light to generate cytotoxic reactive molecular species. The authors begin by discussing the historical context of PDT, highlighting its ancient origins and early applications. They then delve into the fundamental photochemical and photophysical principles underlying PDT, emphasizing the importance of PS fluorescence for both diagnosis and therapy optimization. The review covers various imaging modalities used in conjunction with PDT, including spectroscopy, microscopy, endoscopy, and tomography. Each section focuses on specific applications of imaging in PDT, such as tumor margin resection, treatment monitoring, and dosimetry. The authors also discuss the potential of combining multiple imaging modalities and contrast agents to enhance therapeutic outcomes. A significant portion of the review is dedicated to the use of PS fluorescence detection (PFD) for disease detection and surgical guidance. Early studies on PS fluorescence, particularly with porphyrins like hematoporphyrin derivative (HpD), laid the groundwork for more advanced applications. The introduction of 5-aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX) as a photosensitizer has improved tumor selectivity and imaging performance. The review highlights the clinical impact of PFD in detecting and guiding resection of bladder cancer, brain cancer, and ovarian cancer. For bladder cancer, ALA-induced PpIX has shown high sensitivity and specificity in detecting cancerous lesions, leading to improved recurrence rates. In brain cancer, PFD has enabled more precise tumor resection, improving progression-free survival. For ovarian cancer, PFD has been promising in early detection and monitoring treatment responses. The authors conclude by discussing future directions, emphasizing the need for further research in multiplexing imaging modalities and developing online monitoring techniques to enhance the effectiveness of PDT. They also highlight the potential of combining imaging with other therapeutic approaches to improve treatment outcomes.