Photodynamic therapy (PDT) has been used in ophthalmology for the last 10 years particularly for disorders of the posterior pole. In recent years PDT has been increasingly applied for corneal cross-linking in progressive keratoconus. The classical PDT is performed with porphyrins as photosensitizers and illumination with visible light of 630-635 nm wavelength and cross-linking with the photosensitizer riboflavin and ultraviolet illumination at 370 nm. Illumination of photosensitizers generates free oxygen radicals, which damage the cell membrane or nucleic acids of eukaryotic cells or even microorganisms. By cross-linking a stronger network of collagen fibers can additionally be achieved. Experimental studies have shown that PDT with higher concentrations of photosensitizers may induce necrosis and apoptosis of corneal cells and that survival of herpes simplex virus will be reduced on a LogMar scale by 4-5 lines, of Staphylococcus aureus, Pseudomonas aeruginosa or Candida albicans strains by 1-2 lines. Previous clinical studies have shown that PDT may heal bacterial or even acanthamoeba keratitis. Thus, some authors claim that photodynamic therapy may be a potential alternative in therapy resistant infectious keratitis. However, the limitations of this therapeutic option in particular need further investigation.