Activation of certain chemicals with light leads to photochemical processes that involve the generation of free radicals and excited oxygen species. These molecular species are capable of destroying cells and tissues at or near the areas of photochemistry initiation. An advantage of this approach is that it provides the potential of double selectivity; first, due to the confinement of sites of illumination, and second, due to preferential localization of the activatable chemicals in many neoplastic tissues. Selectivity is further enhanced by delivering the photoactivatable molecules via vehicles such as antibodies and liposomes. The mechanisms of tissue destruction by photochemical processes are different from those used by most cheomtherapeutic drugs. This provides the possibility of using targeted photocemistry in combination with conventional chemotherapy to: 1) destroy cells that are resistant (acquired or intrinsic), and 2) achieve better (broader) tumor response due to the more varied arsenal for attack provided by the combination of photochemical and chemotherapeutic approaches. The best known biomedical application of photochemical targeting is photodynamic therapy (PDT). Research in our laboratories is focused on the mechanistic, therapeutic, and imaging aspects of photobiology to cancer.