Ultraviolet (UV) radiation, occupying the electromagnetic spectrum between visible light and X-rays (100–400 nm), is a pervasive environmental factor with both beneficial and harmful biological effects. While constituting only about 2% of the solar radiation reaching Earth, UV radiation significantly influences human health and ecological systems. Natural and artificial sources of UV —including the sun, medical phototherapy devices, and industrial equipment—emit radiation in varying wavelengths classified as UVC, UVB, and UVA. These types differ in energy, penetration depth, and biological impact. UVC is the most energetic and is largely absorbed by atmospheric ozone, while UVB and UVA reach the Earth’s surface in varying degrees, contributing to vitamin D synthesis but also inducing DNA damage, photoaging, and carcinogenesis. The increasing incidence of UV-related health conditions, such as skin cancers and ocular disorders, underscores the importance of understanding UV radiation’s interaction with biological tissues. Moreover, the depletion of the ozone layer has heightened concerns over rising exposure levels. This chapter provides a comprehensive overview of UV radiation’s classification, sources, and biological implications, emphasizing the need for balanced exposure and protective measures to mitigate its adverse effects.