Photodynamic therapy (PDT) is a versatile treatment with diverse applications in dermatology. PDT combines photosensitizers (PS), most commonly 5-aminolevulinic acid or methyl aminolevulinate, and a light source, such as light-emitting diodes, fluorescent bulbs, lasers, flash lamps, or sunlight, in the presence of molecular oxygen to induce therapeutic effects primarily through singlet oxygen and reactive oxygen species generation. Downstream cellular and physiological effects include apoptosis, necrosis, and immune modulation. PDT efficacy depends on PS parameters, including PS type, concentration, dosing, temperature, and incubation time, and light source parameters such as light source, power density, wavelength, and fluence. PDT is generally safe and well tolerated; potential adverse effects such as pain and erythema are typically mild and self-limiting. Part I of this CME provides a foundational overview of PDT principles, including mechanisms of action, PS, and light sources.