True and precise routine measurements of quantities of clinical interest are essential if results are to be optimally interpreted for patient care. Additionally, results produced by different measurement procedures for the same measurand must be comparable if common diagnostic decision values and clinical research findings are to be broadly applied. Metrology, the science of measurement, provides laboratory medicine with a structured approach to the development and terminology of reference measurement systems which, when implemented, improve the accuracy and comparability of patients' results. The metrological approach is underpinned by the concepts of common measurement units, traceability of measured values, measurement uncertainty and commutability. Where traceability to the International System of Units (SI units) is not yet realized for a measurand, result comparability may be achievable by other, less ideal, approaches. Measurements are the core activity of clinical laboratories, and clinical biochemists should ensure that patients' results are traceable to the highest available reference. This review introduces and illustrates the principles of metrological traceability, describes its critical importance to improving the quality of patients' results and highlights the need to actively promote traceability in clinical laboratories.