For nearly 100 yr physicians and scientists have appreciated the carcinogenic potential of the ultraviolet (UV) radiation present in sunlight 0(1,2). During the latter part of the twentieth century, immunologists and dermatologists realized that UV radiation suppressed the immune response (3–5). Moreover, the immune suppression induced by UV radiation is a major risk factor for the induction of nonmelanoma skin cancer (6). The association between nonmelanoma skin cancer induction and immune suppression has fueled the efforts of many to study the immunologic mechanism underlying UV-induced immune suppression. This chapter focuses on describing the methods used to dissect the suppressive effects of UV on the immune system, concentrating particularly on in vivo models of immunity.