Adapting the international system of units to the 21st century

It is well known that the current definition of the kilogram in the SI, which is defined by reference to a prototype artifact, is unsatisfactory due to the fact that the mass of the prototype is drifting. Some of the other definitions of the base units of the SI are also due for revision and improvement. In October 2011 the CGPM passed a resolution approving plans for new definitions of the kilogram, ampere, kelvin, and mole with the intention of adapting and improving the SI by using the values of fundamental constants (constants of nature) to define the units of the SI. In this paper I shall discuss the reasoning behind both the revised definitions and the revised presentation chosen for the new SI.