The recently updated formal definition of Uncertainty in Measurement (MU) is doubt about the true value of the measurand that remains after making a measurement. This means that the true result for a measurand cannot be exactly known. This paper discusses important aspects of estimating measurement uncertainty (MU) for scientists and technical staff working in medical laboratories. Measurement uncertainty is not concerned with total measurement error. MU is concerned only with the uncertainties introduced by the measuring process itself, for example it starts with primary tube sampling or sample preparation through to result output. MU focuses on defining a range of results that could be obtained for an analyte if a sample was measured repeatedly, providing a quantitative estimate of where the true value of a measured analyte is believed by the laboratory to lie, with a stated confidence level. A medical laboratory’s knowledge of the MU of their reported results provides them with a valuable quality tool. An estimate of the measurement uncertainty of a test result provides a quantitative measure of the reliability of the reported result to demonstrate that the laboratory is meeting, exceeding or failing the reliability performance required by clinical users. The MU estimate can also assist with identifying technical steps in the measurement procedure which significantly contribute to the uncertainty of the measurement procedure’s results. This may also provide the laboratory with the opportunity to reduce the MU for that measurement procedure by modifying or replacing the technical step.