Photodynamic therapy (PDT) is a light activated chemotherapy that is dependent on three parameters: photosensitizer (PS) concentration; oxygen concentration; and light dosage. Due to highly variable treatment response, the development of an accurate dosimeter to optimize PDT treatment outcome is an important requirement for practical applications. Singlet oxygen is an active species in PDT, and we are developing two instruments, an ultra-sensitive singlet oxygen point sensor and a 2D imager, with the goal of a real-time dosimeter for PDT researchers. The 2D imaging system can visualize spatial maps of both the singlet oxygen production and the location of the PS in a tumor during PDT. We have detected the production of singlet oxygen during PDT treatments with both in-vitro and in-vivo studies. Effects of photobleaching have also been observed. These results are promising for the development of the sensor as a real-time dosimeter for PDT which would be a valuable tool for PDT research and could lead to more effective treatment outcome. We summarize recent results in this paper.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.