SummaryTargeted genome‐editing technology using designed nucleases has been evolving rapidly, and its applications are widely expanding in research, medicine and biotechnology. Using this genome‐modifying technology, researchers can precisely and efficiently insert, remove or change specific sequences in various cultured cells, micro‐organisms, animals and plants. This genome editing is based on the generation of double‐strand breaks (DSBs), repair of which modifies the genome through nonhomologous end‐joining (NHEJ) or homology‐directed repair (HDR). In addition, designed nickase‐induced generation of single‐strand breaks can also lead to precise genome editing through HDR, albeit at relatively lower efficiencies than that induced by nucleases. Three kinds of designed nucleases have been used for targeted DSB formation: zinc‐finger nucleases, transcription activator‐like effector nucleases, and RNA‐guided engineered nucleases derived from the bacterial clustered regularly interspaced short palindromic repeat (CRISPR)–Cas (CRISPR‐associated) system. A growing number of researchers are using genome‐editing technologies, which have become more accessible and affordable since the discovery and adaptation of CRISPR‐Cas9. Here, the repair mechanism and outcomes of DSBs are reviewed and the three types of designed nucleases are discussed with the hope that such understanding will facilitate applications to genome editing.