This article is about the development of the metric system from its origins at the time of the French Revolution to the present day. In November 2018, the 26th General Conference on Weights and Measures (CGPM) will be invited to adopt a new definition of the International System of Units, SI, based on fixed numerical values of a set of seven defining constants, broadly the fundamental constants of physics. From these, new definitions of the seven base units of the SI will be deduced. It will then be a little more than two hundred years since a Committee of the l'Académie Royale des Sciences made a proposal to base a new unit of length on a fraction of the meridian of Paris and thereby initiated the creation of the metric system. The redefinition in 2018 will at last put into practice their original proposal for a system independent of time and place, accessible to all and belonging to no one nation. The key is the new possibility of replacing the present definition of the kilogram, an artefact of platinum-iridium kept at the International Bureau of Weights and Measures at Sèvres, by one based on a fixed numerical value of a fundamental constant, the Planck constant.