Synopsis: At the origin of the metric system was the idea of units of measurement based on invariant quantities of nature. After more than 200 years we are now within reach of achieving this. Plans now exist to redefine the kilogram, still defined as the mass of a Pt-Ir cylinder kept in a vault at the BIPM, by fixing the numerical value of the Planck constant h; and the ampere, kelvin and mole by fixing numerical values for e, k and N_A. With the metre already being defined by the speed of light and the second by an atomic microwave transition, we shall have at last achieved what the savants of the 18th century had sought.

The Discussion Meeting held at the Royal Society on 24 and 25 January 2011 reviewed the relation of the International System of Units to the fundamental constants of physics and progress towards the redefinitions. The proceedings will be published in a future issue of Philosophical Transactions A, and copies of the presentations can be downloaded here.

	Presentation	Author
	Adapting the International System of Units to the 21st century	Prof. Ian Mills FRS (University of Reading)
	The Avogadro constant: counting atoms in a single-crystal 28Si sphere	Dr Peter Becker (PTB)
	The watt balance: determination of the Planck constant and redefinition of the kilogram	Dr Michael Stock (BIPM)
	The quantum Hall effect and metrology	Prof. Klaus von Klitzing ForMemRS (Max Planck Institute for Solid State Research, Germany)
	The role of the international prototype of the kilogram after the redefinition of the SI units	Dr Richard Davis (BIPM)
	The concept of mass in Relativistic Quantum Theory	Prof. Christian Bordé (Académie des Sciences, France)
	A new definition for the mole based on the Avogadro constant: a journey from physics to chemistry	Dr Martin Milton (NPL)
	SI and non-SI units for biological quantities	Dr Adrian Bristow (NIBSC)
	The Boltzmann constant from the speed of sound in a not quite spherical cavity	Dr Laurent Pitre (LNE)
	Accurate radiometry from space: an essential tool for climate studies	Dr Nigel Fox (NPL)
	Spectroscopy of muonic hydrogen: determining the proton charge radius	Dr François Nez (LKB)
	Ultracold atoms and precise time standards	Dr William D. Phillips and Dr Gretchen Campbell (NIST)
	Learning from length, time and the redefined SI metre	Dr John L. Hall (JILA)
	When should we change the definition of the second?	Prof. Patrick Gill (NPL)
	Time scales in the context of General Relativity	Dr Bernard Guinot (Paris Observatory)
	Final discussion and overview	Dr Terry Quinn (Emeritus Director BIPM)

More details of the Discussion Meeting can be found at http://royalsociety.org/new-SI/.