Photodynamic therapy (PDT) was originally developed for the treatment of solid tumors by the concerted action of a tumor-localizing agent, named as the photosensitizer, molecular oxygen and visible light source. The photoinduced cytotxic species are mainly represented by singlet oxygen ( 1 O 2 ), even though the formation of NO has been observed in selected situations. Optimal results are generally obtained by using porphyrins and their tetrapyrrolic analogues as the photodynamic agents, since these compounds display an intense absorbance of the red spectra wavelengths, which are endowed with a particularly deep penetration into most biological tissues. While several porphyrins exhibit an intrinsic preferential affinity for malignant tissues, the selectivity of the tumor localization can be significantly enhanced by pre-binding the photosensitizer with a targeting agent, such as an antibody or a vehicle (e.g., glycoproteins, peptides, oligonucleotide aptamers, growth factors, lipoproteins) directed against antigens or receptors which are specifically present at the surface of tumor cells. Recently, novel perspectives have been opened by the association of the photosensitizer with nanoparticles, which can be made to be multifunctional, thereby allowing the simultaneous delivery of photoactivatable moieties acting by different mechanisms, as well as of phototherapeutic and photodiagnostic agents. The chemical structure of the photosensitizer and the nature of the possible carrier have important consequences with regards to the distribution of the photosensitizer among different compartments of the tumor tissue, such as neoplastic cells, blood vessels or the non-vascular stroma, as well as its localization in different subcellular sites; these would obviously affect the mechanism of photoinduced tumor damage, modulating the competition between necrosis, apoptosis and autophagia, the importance of photoinduced hypoxia, and the balance between enhancement of the immune response and immunosuppression. So far, about 250 randomized clinical trials have been officially reported for 4 V. Rapozzi and G. Jori PDT of tumors, and essentially all types of solid tumors with the exception of melanotic melanoma have been found to be positively responsive to the photodynamic treatment. Thus, PDT is currently considered as a reasonable option for the treatment of a variety of malignant lesions for curative or palliative purposes by using either external or endoscopic irradiation approaches via non-coherent or laser light sources.