Abstract: A system and method for monitoring photodynamic therapy of a target tissue, where the target tissue contains a photosensitizing substance, include a first light source configured to deliver light to the target tissue, the first light source having a wavelength capable of exciting the photosensitizing substance. An ultrasonic transducer receives photoacoustic signals generated due to optical absorption of light energy by the target tissue, and a control unit in communication with the ultrasonic transducer reconstructs photoacoustic tomographic images from the received photoacoustic signals to provide an indication of optical energy deposition due to the photosensitizing substance in the target tissue.