Photodynamic therapy (PDT) uses photosensitizers (PS) and light to treat diseases. PS are non-toxic dyes that, when activated by light, generate reactive oxygen species that destroy cells. PDT has been used for over a century but is now widely applied. It was originally developed for cancer treatment but is now used for non-malignant diseases. This review discusses the mechanisms of PDT, focusing on PS, photochemistry, and cellular localization. PS are typically derived from tetrapyrrole structures like heme, chlorophyll, and bacteriochlorophyll. They have specific absorption properties that allow them to penetrate tissues. PS can be localized in various subcellular compartments, such as mitochondria, lysosomes, endoplasmic reticulum, Golgi apparatus, and plasma membrane. The localization of PS determines their effectiveness in PDT, as it influences how they interact with cells and tissues. The photochemistry of PS involves two main pathways: Type I (radicals and reactive oxygen species) and Type II (singlet oxygen). The efficiency of PDT depends on the PS's ability to generate these species and their interaction with cellular components. PS that are fluorescent can be used for imaging and monitoring during PDT. Recent advances in PS development have led to a wide variety of compounds, making it challenging to choose the most suitable one for a particular application. Factors such as PS structure, solubility, and stability are important in selecting an effective PS. Additionally, the use of 5-aminolevulinic acid (ALA) as a precursor for protoporphyrin IX (PPIX) has been explored for PDT applications. Light delivery is crucial for effective PDT, as it determines how well the PS reaches the target tissue. The optical properties of tissues, including absorption and scattering, influence light penetration. PS that absorb light in the red or far-red wavelengths are more effective for deep tissue penetration. The subcellular localization of PS is a key factor in PDT success. PS that localize in mitochondria, for example, can cause mitochondrial damage and apoptosis. PS that localize in lysosomes may lead to photodamage and cell death. The localization of PS also affects their ability to be detected and monitored during PDT. In conclusion, PDT is a complex therapy that depends on the properties of PS, light delivery, and tissue characteristics. Understanding the mechanisms of PDT, including PS photochemistry, cellular localization, and light delivery, is essential for optimizing its effectiveness and minimizing side effects. The development of new PS and improvements in light delivery techniques continue to enhance the potential of PDT for treating a wide range of diseases.Photodynamic therapy (PDT) uses photosensitizers (PS) and light to treat diseases. PS are non-toxic dyes that, when activated by light, generate reactive oxygen species that destroy cells. PDT has been used for over a century but is now widely applied. It was originally developed for cancer treatment but is now used for non-malignant diseases. This review discusses the mechanisms of PDT, focusing on PS, photochemistry, and cellular localization. PS are typically derived from tetrapyrrole structures like heme, chlorophyll, and bacteriochlorophyll. They have specific absorption properties that allow them to penetrate tissues. PS can be localized in various subcellular compartments, such as mitochondria, lysosomes, endoplasmic reticulum, Golgi apparatus, and plasma membrane. The localization of PS determines their effectiveness in PDT, as it influences how they interact with cells and tissues. The photochemistry of PS involves two main pathways: Type I (radicals and reactive oxygen species) and Type II (singlet oxygen). The efficiency of PDT depends on the PS's ability to generate these species and their interaction with cellular components. PS that are fluorescent can be used for imaging and monitoring during PDT. Recent advances in PS development have led to a wide variety of compounds, making it challenging to choose the most suitable one for a particular application. Factors such as PS structure, solubility, and stability are important in selecting an effective PS. Additionally, the use of 5-aminolevulinic acid (ALA) as a precursor for protoporphyrin IX (PPIX) has been explored for PDT applications. Light delivery is crucial for effective PDT, as it determines how well the PS reaches the target tissue. The optical properties of tissues, including absorption and scattering, influence light penetration. PS that absorb light in the red or far-red wavelengths are more effective for deep tissue penetration. The subcellular localization of PS is a key factor in PDT success. PS that localize in mitochondria, for example, can cause mitochondrial damage and apoptosis. PS that localize in lysosomes may lead to photodamage and cell death. The localization of PS also affects their ability to be detected and monitored during PDT. In conclusion, PDT is a complex therapy that depends on the properties of PS, light delivery, and tissue characteristics. Understanding the mechanisms of PDT, including PS photochemistry, cellular localization, and light delivery, is essential for optimizing its effectiveness and minimizing side effects. The development of new PS and improvements in light delivery techniques continue to enhance the potential of PDT for treating a wide range of diseases.