Metrological traceability is the backbone of all measurements of all types, from weighing ingredients for a meal to measuring the speed of light. In the clinical laboratory the SI system underpins nearly all aspects such as sample volumes, weighing out standards, light intensity measurements and timing a reaction. The SI system is overseen by the International Bureau for Weights and Measures (BIPM) in Paris and aims to support “the same result, everywhere, everytime”. The concepts of metrological traceability are also directly used for laboratory measurements themselves with reference materials and reference methods having been introduced in many fields over the years. It can be said that all laboratory measurements are made by comparison (with a calibrator or other reference) although this is less clear for many common haematology tests. Recently there has been actions to join the concepts of metrology (the study of measurement), represented by the BIPM, with laboratory medicine, represented by the International Federation of Clinical Chemistry (IFCC), by forming the JCTLM (Joint Committee for Traceability in Laboratory Medicine). Plans are underway to include the International Council for Standardization in Haematology (ICSH) in this organisation. The aim is to provide equivalent results for laboratory tests over time and place, a requirement for modern medicine where research, diagnostic companies and patients are all global entities.