The electrophoretic mobility shift assay (EMSA) can be used to study proteins that bind to DNA structures created by DNA-damaging agents. UV-damaged DNA-binding protein (UV-DDB), which is involved in nucleotide excision repair, binds to DNA damaged by ultraviolet radiation or the anticancer drug cisplatin. Ku, XRCC4/Ligase IV, and DNA-PKcs, which are involved in the repair of DNA double-strand breaks by nonhomologous end joining, assemble in complexes at DNA ends. This chapter will describe several EMSA protocols for detecting different DNA repair protein-DNA complexes. To obtain additional information, one can apply variations of the EMSA, which include the reverse EMSA to detect binding of (35)S-labeled protein to damaged DNA, and the antibody supershift assay to detect the presence of a specific protein in the protein-DNA complex.