Biological markers of radiation exposure play a crucial role in the triage of suspected exposed persons following a radiation accident or incident. They can also estimate individual doses that enable assessment of late radiation effects in affected individuals. In recent years the gene expression assay has been shown to be a sensitive biological marker of radiation exposure with the potential to be used for truly individualised dosimetry. The possibility for this gene expression assay to be used in a large scale mass-casualty scenario has been proposed and tested in a recent intercomparison exercise. Classic cytogenetic techniques, and in particular the gold standard dicentric assay, have two main disadvantages: (1) lack of high-throughput and (2) delays of several days between blood sampling and the availability of results. Although more work needs to be done to further assess its suitability for triage purposes, it is clear that gene expression analysis in blood samples can provide valuable information, as there is a window of time (i.e. 12-48 hours) following radiation exposure where specific radiation-responsive genes have linear dose responses (0-5 Gy). Most work to date has focused on developing sensitive assays for studying gene expression modifications using state of the art technology, i.e. multiplex quantitative, digital polymerase chain reaction (qPCR) and molecular counting systems.