Phototherapy successfully uses the short-term effects of ultraviolet light against inflammation and proliferation. For its long-term effects, however, ultraviolet light was recently classified as a carcinogen. The wave spectrum employed in phototherapy has various carcinogenic effects in experimental systems, most notably DNA mutations in keratinocytes. Clinically, PUVA increases the risk for squamous cell carcinoma of the skin, especially after following 350 or more phototherapy sessions over a lifetime. Melanoma and genital skin cancer are not increased by PUVA alone. Previous UV damage, immunosuppression and other systemic treatments increase cutaneous carcinogenesis through PUVA. In contrast, broad-band UVB, narrow-band UVB and UVA1 have not yet been linked to cutaneous carcinogenesis, but will need careful follow-up in larger studies. Phototherapy remains a safe treatment modality, provided that the indication is well-founded, previous exposure and co-carcinogens are considered, and short and dose-intensive treatment protocols are favored, PUVA is chosen as second-line treatment that should not be used for more than a lifetime total of 250-300 phototherapy sessions.