At the initiation of DNA double-strand break repair, a number of ubiquitylation events occur; here, the RNF8 ubiquitin E3 ligase and the ubiquitin-conjugating E2 enzyme, UBC13, are shown to primarily modify H1-type linker histones, via a K63 linkage. At the initiation of DNA double-strand break repair, a number of ubiquitination events occur. One damage-related ubiquitin E3 ligase, RNF168, is known to modify H2A family histones, but the target of the RNF8 ubiquitin E3 ligase was not clear. Niels Mailand and colleagues have now determined that RNF8 and the ubiquitin-conjugating E2 enzyme, UBC13, primarily modify H1-type linker histones, via a K63 linkage. This modification itself recruits RNF168 and downstream repair factors to the break. DNA double-strand breaks (DSBs) are highly cytotoxic DNA lesions that trigger non-proteolytic ubiquitylation of adjacent chromatin areas to generate binding sites for DNA repair factors. This depends on the sequential actions of the E3 ubiquitin ligases RNF8 and RNF168 (refs 1, 2, 3, 4, 5, 6), and UBC13 (also known as UBE2N), an E2 ubiquitin-conjugating enzyme that specifically generates K63-linked ubiquitin chains7. Whereas RNF168 is known to catalyse ubiquitylation of H2A-type histones, leading to the recruitment of repair factors such as 53BP1 (refs 8, 9, 10), the critical substrates of RNF8 and K63-linked ubiquitylation remain elusive. Here we elucidate how RNF8 and UBC13 promote recruitment of RNF168 and downstream factors to DSB sites in human cells. We establish that UBC13-dependent K63-linked ubiquitylation at DSB sites is predominantly mediated by RNF8 but not RNF168, and that H1-type linker histones, but not core histones, represent major chromatin-associated targets of this modification. The RNF168 module (UDM1) recognizing RNF8-generated ubiquitylations11 is a high-affinity reader of K63-ubiquitylated H1, mechanistically explaining the essential roles of RNF8 and UBC13 in recruiting RNF168 to DSBs. Consistently, reduced expression or chromatin association of linker histones impair accumulation of K63-linked ubiquitin conjugates and repair factors at DSB-flanking chromatin. These results identify histone H1 as a key target of RNF8–UBC13 in DSB signalling and expand the concept of the histone code12,13 by showing that posttranslational modifications of linker histones can serve as important marks for recognition by factors involved in genome stability maintenance, and possibly beyond.