Maintaining accurate laboratory measurements over time is crucial for assuring appropriate patient care and disease management. Accurate results over time and location are achieved by standardising measurements and establishing traceability to a reference system. Reference materials are key components of such reference systems and for establishing traceability. Commutability of reference materials is a critical property to ensure they are fit for use. Commutability is defined as the equivalence of the mathematical relationships between the results of different measurement procedures for a reference material and for representative samples from healthy and diseased individuals. This material characteristic is of special importance for measurement procedures that are optimised for measuring analytes directly in patient samples. The commutability of a reference material is measurement procedure specific and its assessment requires special experimental designs. This review explains the importance of commutability and summarises different experimental approaches described in the literature that have been used to assess the commutability of reference materials in clinical chemistry.