Diaphanoscopy is an almost 150-year-old diagnostic procedure based on the transillumination of the wall of the eyeball. One of the most important fields of application is the detection of tumors, which can be distinguished from the rest of the tissue by their reduced light transmission. Other fields of application include the diagnosis of retinal tears and holes, the detection of foreign bodies in the vitreous body and the control of coagulative measures in the ciliary body. In comparison to other techniques, diaphanoscopy requires only little effort and is very gentle on patients.