Because of a substantial rise in the incidence of skin cancer in the United Kingdom and elsewhere a greater awareness of the role of sun-induced cutaneous genetic damage has developed. This, in turn, has increased interest in the cellular mechanisms responsible for tumorigenesis, and the need to develop experimental methodologies to investigate these mechanisms. DNA represents a most important cellular target for ultraviolet radiation (UVR), leading to the formation of various DNA damage products. A number of these products, such as the cyclobutane pyrimidine dimer, have been implicated in the pathogenesis of various UVR-related conditions. In this chapter we detail a number of methods for assessing UVR-induced DNA damage using two antisera which recognize cyclobutane thymine dimers (T-T). Immuno-approaches have a number of benefits over chromatographic techniques, and have been applied herein to quantitatively and qualitatively assess the presence of T-T in cultured keratinocytes, human skin, and urine, providing information about lesion induction and repair.