While immunoassays are now used in a variety of fields including medical, food technology and environmental protection because of their high sensitivity, high specificity and ease of automation to provide highly cost effective analyses, it has been in medical testing that their standardization has been most intensely developed. The use of internationally agreed clinical protocols, common reference intervals or decision limits and even electronic health records across health-care institutions are all dependent on medical testing laboratories performing assays that are traceable to internationally recognized reference measurement systems. The application of metrological principles to achieve traceability and standardization for immunoassays is being pursued to this end. Comprehensive measurement systems are available for the total serum hapten assays currently measured in the clinical laboratory by immunoassay. Current research is investigating the usefulness of developing defined systems for the measurement of non-bound fractions or "free" hormone levels of these types of analytes. One strategy that has been successfully applied for the standardization of assays for large molecular weight polypeptide measurands has been to localise the biological activity to a small molecular weight moiety of the meausurand and to establish a reference measurement procedure for this moiety as a surrogate for the total molecule of clinical interest. The standardization or harmonization of assays for heterogeneous polypeptide hormones is also is another area of current research for standardization of clinical immunoassays. The accreditation of medical testing laboratories to ISO15189 standard requires that all testing methods including immunoassays are validated or verified to be fit for purpose including the accuracy of the assay and the estimation of measurement uncertainty as assessed by traceability to certified reference materials through a documented unbroken chain of calibrations.