Many cancer therapeutics, including radiation therapy, damage DNA eliciting the DNA damage response (DDR). Clinical assays that characterise the DDR could be used to personalise cancer treatment by indicating the extent of damage to tumour and normal tissues and the nature of the cellular response to that damage. The phosphorylated histone γH2AX is generated early in the response to DNA double-strand breaks, the most deleterious form of DNA damage. Translational researchers are developing tissue sampling and assay strategies to apply the measurement of γH2AX to a range of clinical questions, including that of tumour response. The presence of γH2AX is also associated with other cell states including replication stress, hypoxia and apoptosis, which could influence the relationship between γH2AX and clinical endpoints. This review aims to assess the potential of γH2AX as a practical and clinically useful biomarker of tumour and normal tissue responses to therapy.