Photodynamic therapy (PDT) is an emerging treatment modality that employs the photochemical interaction of three components: light, photosensitizer, and oxygen. Tremendous progress has been made in the last 2 decades in new technical development of all components as well as understanding of the biophysical mechanism of PDT. The authors will review the current state of art in PDT research, with an emphasis in PDT physics. They foresee a merge of current separate areas of research in light production and delivery, PDT dosimetry, multimodality imaging, new photosensitizer development, and PDT biology into interdisciplinary combination of two to three areas. Ultimately, they strongly believe that all these categories of research will be linked to develop an integrated model for real-time dosimetry and treatment planning based on biological response.