Ultraviolet Radiation (UV) has been shown to suppress immunological parameters in both animal models and humans. It has been postulated that this UV induced immune suppression may lead to an increase in susceptibility to infectious diseases and allow the outgrowth of transformed tumours leading to the development of skin cancer. The aim of the research presented in this book was to further understand the mechanisms of UV induced immune suppression in both mice and humans. The research focuses on the role of urocanic acid, cytokines, and wavelength interactions and examines their involvement in UV mediated immune suppression. This book is essentially the publication of a PhD thesis and would appeal to research scientists, clinicians and students working in the field of photoimmunology or skin biology.