Photosensitized reactions are powerful tools to study cellular processes and macromolecular interactions. As therapeutic agents, photosensitizers are becoming increasingly significant in the treatment of a growing number of diseases including cancer. Moreover, chemical conjugation of photosensitizers to highly-selective receptor-specific probes, including monoclonal antibodies, promises to improve the effectiveness, selectivity and lethality of these photoactive agents. Photosensitizer-probe conjugates can site-direct photoreactions to specific cells, cell receptors and/or intracellular targets. The relatively low number of adverse effects associated with this new generation of site-directed photosensitive agents will lead to novel applications in research, photodynamic therapy and cancer chemotherapy. This chapter discusses photosensitizer agents, photoreactions, chemical conjugation to specific probes and potential applications of photosensitizer- probe conjugates.