In the century since Paul Erlich's innovative immunological standardization work with diphtheria anti-toxin, the field of immunological standardization has expanded dramatically. Biological standards for a diverse range of immunological substances have been produced e.g. immunoglobulins, complement components, autoantibodies and blood group reagents. The concept of calibration of such materials in biological units of potency is now widely accepted for many such substances. Most recently much effort has been devoted to producing biological standards for cytokines which can be used to calibrate and validate biological assays for these analytes. Immunoassays have been found to be particularly problematical from the standardization view point although provision of a single international standard for distribution world-wide is clearly advantageous and helps reduce assay variability. It is hoped that the considerable progress with immunological standardization achieved during the past century will continue and expand to ensure the validity of existing and new immunological assays which will be required in the future.