Despite its more than 100-year history in experimental and clinical use, photodynamic therapy (PDT) is only starting to be appreciated for its full potential. PDT combines a photosensitizer and light in the presence of oxygen to treat cancer and other disorders. This paper reviews the molecular mechanism of PDT at the cellular level as well as in therapeutic settings in vivo. The availability of multiple photosensitizers with different structures and functional properties makes PDT an extremely versatile and, conversely, a challenging approach to cancer therapy. The advancing understanding of molecular pathways helps to design improved regimens. As most cancers are being treated with combined therapies, PDT is being integrated into rationally designed regimens that exploit molecular responses to PDT for improved efficacy.