Photodynamic therapy (PDT) is fighting cancer with light. What might sound strange at first glance is a very potent instrument, which can replace surgery in many cases. The principle is rather simple. In a first step, so-called photosensitizers are applicated by infusion. These agents are non-toxic but accumulate very specifically in cancerous tissue. Second step is to place laser catheters in or next to the tumor. By using a specific wavelength of the laser, the photosensitizers are activated and become toxic to cancer cells. While healthy tissue around the tumor is not affected by the procedure, the impact on the tumor is impressive. Originally only performed on superficial skin tumors, modern approaches include endoscopic application of laser light. Therefore, the PDT can be used for a wide range of different tumors and locations, comprising stomach, liver, gall bladder, pancreatic gland, gut, bones, ovaries, prostate gland, urinary bladder, uterus and even intracranial tumors. The treatment is tolerated well; patients should avoid direct sunlight for a couple of days after the treatment.