DNA double-strand breaks (DSBs) represent the most toxic form of DNA damage and can arise in either physiological or pathological conditions. If left unrepaired, these DSBs can lead to genome instability which serves as a major driver to tumorigenesis and other pathologies. Consequently, localizing DSBs and understanding the dynamics of break formation and the repair process are of great interest for dissecting underlying mechanisms and in the development of targeted therapies. Here, we describe END-seq, a highly sensitive next-generation sequencing technique for quantitatively mapping DNA double-strand breaks (DSB) at nucleotide resolution across the genome in an unbiased manner. END-seq is based on the direct ligation of a sequencing adapter to the ends of DSBs and provides information about DNA processing (end resection) at DSBs, a critical determinant in the selection of repair pathways. The absence of cell fixation and the use of agarose for embedding cells and exonucleases for blunting the ends of DSBs are key advances that contribute to the technique's increased sensitivity and robustness over previously established methods. Overall, END-seq has provided a major technical advance for mapping DSBs and has also helped inform the biology of complex biological processes including genome organization, replication fork collapse and chromosome fragility, off-target identification of RAG recombinase and gene-editing nucleases, and DNA end resection at sites of DSBs.