For more than two decades, chromosome aberration analysis has been used in biomonitoring of occupational and environmental exposure to ionising radiation. Chromosome aberration analysis is a method used to detect unstable aberrations in the lymphocytes of irradiated personnel. In turn, stable chromosome aberrations that arise some time after exposure are detected using the multicolour fluorescence in situ hybridisation. This is a technique which dyes each pair of chromosomes with different colour. Due to the dynamics of unstable aberration formation, chromosome aberration analysis is more suitable for genome damage assessment of recent exposures. On the other hand, fluorescence in situ hybridisation gives information on chromosome instability caused by long-time occupational exposure to ionising radiation. Considering the high cost of fluorescence in situ hybridisation and the uncertainty of the result, it should be used in biodosimetry only when it is absolutely necessary.