Chromosomal aberration analysis is the most sensitive biological method to indicate exposure to ionising radiation. This paper will distinguish between the detection and the measurement of low doses by aberration analysis and show how to quantify their limits. Worked examples will be presented using the lymphocyte dicentric assay and data typical of Co-60 gamma rays. The principles illustrated can be applied to other aberration types and other radiation qualities. Two situations will be considered: conventional by eye scoring of 1,000 metaphases from a suspected low exposure patient and scoring more metaphases with computer assisted microscopy. Low dose quantification is ultimately limited by the uncertainty on the assumed background level of dicentrics. With conventional scoring the Poisson uncertainty on the patient's observed dicentric frequency is the major component to the uncertainty on low dose estimates. With increased scoring, assisted by computer, this is reduced but the standard error on the linear calibration coefficient becomes more important. The optimum is reached where both components contribute equally to the overall uncertainty. A dose estimate may be considered as a measurement when its lower 95% confidence limit falls above zero. A dose can be regarded as having been detected when the dicentric frequency is above an assumed background but the lower 95% confidence limit includes zero. Conventional scoring of 1,000 metaphases will permit a measurement lower limit of about 100mGy of gamma radiation. This can be reduced by scoring many more cells (∼10,000) to about 70mGy. Further improvement is unlikely due to the background 'noise' in the assay.