Despite the development of highly effective biologics for skin diseases such as psoriasis or atopic dermatitis, UVA and UVB therapy, alone or in combination, are still essential components of various guidelines. Phototherapy is not only a first-line treatment and highly effective for a number of skin diseases, but is also economical and has few side effects. The targeted use of UVA and UVB, if necessary, in combination with the photosensitizer psoralen in the context of PUVA therapy, enables the dermatologist to effectively treat a wide variety of skin diseases. Indications for phototherapy include epidermal diseases such as atopic dermatitis, psoriasis and vitiligo, as well as photodermatoses, mycosis fungoides, graft-versus-host disease and deep dermal diseases such as scleroderma. This article reviews the physical principles, molecular mechanisms, current treatment regimens, and individual indications for phototherapy and photochemotherapy.