The therapeutic use of ultraviolet radiation for medical purposes has a long history, whether it be sunlight in heliotherapy or artificial lamps in phototherapy, to name a few. The interest in ultraviolet radiation as treatment for various cutaneous diseases increased exponentially in the second half of the twentieth century. UV-based therapies, which include narrowband (NB) UVB, broad-band (BB) UVB, and psoralen and UVA (PUVA), are well-established treatment options for diverse dermatologic conditions such as atopic dermatitis, vitiligo, cutaneous T-cell lymphoma, and psoriasis either as monotherapy or as an adjuvant to systemic therapy. This monography focuses on the principal indications of phototherapy in dermatology.