Porphyrins and many other photosensitizing compounds have been shown to induce cytotoxic effects on cells and tissues. These effects have been utilized to treat several types of neoplastic diseases. The treatment is named photodynamic therapy (PDT) and is based on injection of a photosensitizing and tumorlocalizing dye followed by exposure of the tumor region to light. The cytotoxic effect is mediated mainly through the formation of singlet oxygen. This reactive intermediate has a very short lifetime in cells (<0.04ms). Thus, the primary cytotoxic effect of PDT is executed during light exposure and very close to the sites of formation of singlet oxygen. Singlet oxygen reacts with and oxidizes proteins (histidine, tryptophan, methionine, cysteine, tyrosine), DNA (guanine), unsaturated fatty acids and cholesterol.