DNA repair is essential for cells to maintain genome stability in an environment that constantly produces DNA damage. There is a growing appreciation that defects in homologous recombination repair underlie hereditary and sporadic tumourigenesis, and that deficiency in this pathway may dictate the sensitivity of tumours to certain DNA-damaging agents. Homologous recombination deficiency (HRD) may therefore prove to be a diagnostic criterion per se if appropriate biomarkers become available to identify these tumours. In addition, homologous recombination-deficient tumours are more sensitive to inhibition of other DNA repair pathways through so-called 'synthetic lethal interactions', a principle that is currently being tested in clinical trials. Finally, homologous recombination repair-deficient cells may have an increased dependency on certain cell-cycle checkpoints, which can be therapeutically exploited. Here we describe recent advances in strategies to identify and target HRD tumours, approaches to overcome resistance, and combinatory strategies to optimize treatment outcome.