The formation of DNA photoproducts by ultraviolet (UV) light is responsible for induction of mutations and development of skin cancer. To understand UV mutagenesis, it is important to know the mechanisms of formation and repair of these lesions. Cyclobutane pyrimidine dimers and (6-4)photoproducts are the two major classes of UV-induced DNA lesions. Their distribution along DNA sequences in vivo is strongly influenced by nucleosomes and other DNA binding proteins. Repair of UV photoproducts is dependent on the transcriptional status of the sequences to be repaired and on the chromatin environment. Sensitive techniques are now available to study repair of UV damage at the level of nucleotide resolution in mammalian cells. With the aid of in vitro systems, the entire nucleotide excision repair process has been reconstituted from purified protein components with naked DNA as a substrate. Future work will focus on the development of in vitro assays for transcription-coupled repair and repair in chromatin.