The Metre Convention, the BIPM and the SI

This article outlines the origins and history of the Metre Convention, the BIPM and the International System of Units (SI), with particular reference to the historical development of units based on fundamental constants or invariants of nature. In the past, the ideas and the intention to proceed towards a unit system based on invariants of nature had existed but it has only recently become a practical possibility. The adoption by the 24th General Conference on Weights and Measures in October 2011 of a Resolution outlining the principles of such a system is the culmination of more than two hundred years of advances in physics and metrology.