Photodynamic therapy (PDT) uses photosensitizers (PSs) that absorb light to generate reactive oxygen species, which destroy cancer cells and pathogens. PSs are typically tetrapyrrole structures like porphyrins, chlorins, bacteriochlorins, and phthalocyanines, which have been widely studied and some approved for clinical use. Other PSs include synthetic dyes, transition metal complexes, and natural products such as hypericin, riboflavin, and curcumin. Targeted PDT uses PSs conjugated to ligands that bind to specific receptors. Nanotechnology has enhanced PDT through nanoparticle delivery, fullerene-based PSs, and upconversion nanoparticles. Future directions include photochemical internalization, genetically encoded PSs, theranostics, two-photon absorption PDT, and sonodynamic therapy using ultrasound. Key factors for effective PSs include strong absorption in the red/near-infrared range, high triplet quantum yield, and minimal dark toxicity. PDT has seen renewed interest in antimicrobial applications, with PSs tailored for different targets. Various PS structures, including synthetic dyes and natural products, are being explored for their efficacy in PDT. Advances in nanotechnology and targeted delivery systems are improving the precision and effectiveness of PDT.Photodynamic therapy (PDT) uses photosensitizers (PSs) that absorb light to generate reactive oxygen species, which destroy cancer cells and pathogens. PSs are typically tetrapyrrole structures like porphyrins, chlorins, bacteriochlorins, and phthalocyanines, which have been widely studied and some approved for clinical use. Other PSs include synthetic dyes, transition metal complexes, and natural products such as hypericin, riboflavin, and curcumin. Targeted PDT uses PSs conjugated to ligands that bind to specific receptors. Nanotechnology has enhanced PDT through nanoparticle delivery, fullerene-based PSs, and upconversion nanoparticles. Future directions include photochemical internalization, genetically encoded PSs, theranostics, two-photon absorption PDT, and sonodynamic therapy using ultrasound. Key factors for effective PSs include strong absorption in the red/near-infrared range, high triplet quantum yield, and minimal dark toxicity. PDT has seen renewed interest in antimicrobial applications, with PSs tailored for different targets. Various PS structures, including synthetic dyes and natural products, are being explored for their efficacy in PDT. Advances in nanotechnology and targeted delivery systems are improving the precision and effectiveness of PDT.