The 20th May 2019 marks a particularly special celebration of World Metrology Day because it is the day chosen for the implementation of the changes agreed to the definitions of the SI base units. These landmark changes were decided at the 26th meeting of the General Conference on Weights and Measures (CGPM), which took place in Versailles in November 2018.

From 20 May 2019 the International System of Units (SI) embraces one of the most significant changes since its establishment - the definitions of four units (the kilogram, the ampere, the kelvin and the mole) are also linked to physical constants, which ensure their stability and universality. More information on the impact the redefinition has on the realization of the units is available on the BIPM website.

The world-wide promotional activities surrounding the meeting of the CGPM last November were very successful; we hope that the efforts to generate awareness about the implementation of the decisions will be similarly effective. Many examples of initiatives under way around the world are available from the World Metrology Day resource website (http://www.worldmetrologyday.org/).

Whilst the motivation for the changes to the definitions has been to provide new opportunities to increase access to accurate measurements there are some small changes that come into effect today that will concern laboratories working at the highest levels in the mass and electricity communities. Information about these changes is available on the BIPM website.

We wish success to all activities around the world on World Metrology Day that are raising awareness of how the SI is "Fundamentally Better".