On May 20, 2019, metrology underwent an unprecedented revolution. Four of the seven base units have been redefined so that today the entire system is based on fixed values of fundamental constants. In particular, the unit of mass, the kilogram, is now related to Planck's constant.
After a brief introduction and the presentation of the new International System of units (SI), we will focus on the new realizations of the kilogram, in particular the watt balance also called Kibble balance. The design and operation of the Metas experiment will be explained and its latest results will serve as a conclusion.